Add CharacterValue type

[dsainati1]

Closes #1373

Description

Cadence previously lacked a runtime character value, causing some inconsistency between the static Character type and values with that type at runtime. This adds runtime Character values, which are produced by indexing into a string and by creating a character variable like so: let x: Character = "a".

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[github-actions]

Cadence Benchstat comparison

This branch with compared with the base branch onflow:master commit d3124c5
The command for i in {1..N}; do go test ./... -run=XXX -bench=. -shuffle=on; done was used.
Bench tests were run a total of 7 times on each branch.

Results

	old.txt	new.txt
	time/op		delta
RuntimeResourceDictionaryValues-2	13.7ms ± 4%	14.0ms ± 4%	~	(p=0.165 n=7+7)
RuntimeFungibleTokenTransfer-2	1.10ms ±30%	1.14ms ±27%	~	(p=0.902 n=7+7)
ParseDeploy/byte_array-2	19.5ms ± 2%	19.8ms ± 6%	~	(p=0.383 n=7+7)
ParseFungibleToken-2	175µs ± 5%	174µs ± 6%	~	(p=0.710 n=7+7)
ParseInfix-2	9.25µs ± 3%	9.15µs ± 5%	~	(p=0.945 n=6+7)
ParseArray-2	12.2ms ± 3%	12.1ms ± 4%	~	(p=0.699 n=6+6)
QualifiedIdentifierCreation/Three_levels-2	132ns ± 4%	131ns ± 4%	~	(p=0.402 n=7+7)
CheckContractInterfaceFungibleTokenConformance-2	128µs ± 5%	125µs ± 3%	~	(p=0.731 n=7+6)
NewInterpreter/new_interpreter-2	1.03µs ± 2%	1.00µs ± 5%	~	(p=0.175 n=7+6)
NewInterpreter/new_sub-interpreter-2	2.01µs ± 3%	2.01µs ± 6%	~	(p=0.805 n=7+7)
ParseDeploy/decode_hex-2	1.12ms ± 4%	1.08ms ± 4%	−3.62%	(p=0.038 n=7+7)
InterpretRecursionFib-2	2.39ms ± 6%	2.28ms ± 4%	−4.73%	(p=0.007 n=7+7)
ContractInterfaceFungibleToken-2	39.5µs ± 6%	37.5µs ± 3%	−5.25%	(p=0.026 n=7+7)
QualifiedIdentifierCreation/One_level-2	2.23ns ± 7%	2.07ns ± 0%	−7.01%	(p=0.050 n=7+7)
	alloc/op		delta
CheckContractInterfaceFungibleTokenConformance-2	65.7kB ± 0%	66.0kB ± 0%	+0.44%	(p=0.002 n=6+6)
ContractInterfaceFungibleToken-2	26.5kB ± 0%	26.6kB ± 0%	+0.30%	(p=0.001 n=7+7)
RuntimeFungibleTokenTransfer-2	277kB ± 0%	277kB ± 0%	+0.10%	(p=0.003 n=5+7)
RuntimeResourceDictionaryValues-2	4.05MB ± 0%	4.05MB ± 0%	~	(p=0.318 n=7+7)
QualifiedIdentifierCreation/One_level-2	0.00B	0.00B	~	(all equal)
QualifiedIdentifierCreation/Three_levels-2	64.0B ± 0%	64.0B ± 0%	~	(all equal)
NewInterpreter/new_interpreter-2	768B ± 0%	768B ± 0%	~	(all equal)
NewInterpreter/new_sub-interpreter-2	1.24kB ± 0%	1.24kB ± 0%	~	(all equal)
InterpretRecursionFib-2	1.24MB ± 0%	1.24MB ± 0%	~	(p=1.168 n=6+7)
	allocs/op		delta
ContractInterfaceFungibleToken-2	457 ± 0%	458 ± 0%	+0.22%	(p=0.001 n=7+7)
CheckContractInterfaceFungibleTokenConformance-2	1.07k ± 0%	1.07k ± 0%	+0.19%	(p=0.001 n=7+7)
RuntimeFungibleTokenTransfer-2	4.62k ± 0%	4.62k ± 0%	+0.04%	(p=0.017 n=7+7)
RuntimeResourceDictionaryValues-2	102k ± 0%	102k ± 0%	~	(p=0.906 n=7+7)
QualifiedIdentifierCreation/One_level-2	0.00	0.00	~	(all equal)
QualifiedIdentifierCreation/Three_levels-2	2.00 ± 0%	2.00 ± 0%	~	(all equal)
NewInterpreter/new_interpreter-2	12.0 ± 0%	12.0 ± 0%	~	(all equal)
NewInterpreter/new_sub-interpreter-2	38.0 ± 0%	38.0 ± 0%	~	(all equal)
InterpretRecursionFib-2	25.0k ± 0%	25.0k ± 0%	~	(all equal)

[turbolent]

Looks good, great work!

[codecov-commenter]

Codecov Report

Merging #1392 (eacabaa) into master (d3124c5) will decrease coverage by 2.76%.
The diff coverage is 74.79%.

@@            Coverage Diff             @@
##           master    #1392      +/-   ##
==========================================
- Coverage   75.81%   73.05%   -2.77%     
==========================================
  Files         279      280       +1     
  Lines       38780    38900     +120     
==========================================
- Hits        29402    28419     -983     
- Misses       8016     9038    +1022     
- Partials     1362     1443      +81     

Flag	Coverage Δ
unittests	73.05% <74.79%> (-2.77%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
runtime/interpreter/dynamictype.go	66.66% <0.00%> (-1.91%)	⬇️
runtime/interpreter/hashablevalue.go	100.00% <ø> (ø)
runtime/interpreter/visitor.go	8.33% <0.00%> (-0.22%)	⬇️
runtime/interpreter/decode.go	44.50% <30.76%> (-0.20%)	⬇️
values.go	71.09% <50.00%> (-0.39%)	⬇️
encoding/json/decode.go	71.08% <66.66%> (-0.05%)	⬇️
runtime/interpreter/encode.go	65.72% <66.66%> (+0.01%)	⬆️
runtime/interpreter/value.go	55.95% <75.00%> (-15.34%)	⬇️
encoding/json/encode.go	93.50% <100.00%> (+0.08%)	⬆️
runtime/convertValues.go	77.41% <100.00%> (+0.14%)	⬆️
... and 12 more

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[turbolent]

Nice work! Just a couple minor things and then ready to get merged 👍