Add support for metadata with String keys

[Tokazama]

No description provided.

[codecov]

Codecov Report

Merging #6 (6b9b34a) into master (b183e9b) will increase coverage by 10.10%.
The diff coverage is 84.61%.

@@             Coverage Diff             @@
##           master       #6       +/-   ##
===========================================
+ Coverage   83.66%   93.76%   +10.10%     
===========================================
  Files           7        7               
  Lines         349      353        +4     
===========================================
+ Hits          292      331       +39     
+ Misses         57       22       -35     

Impacted Files	Coverage Δ
src/Metadata.jl	100.00% <ø> (ø)
src/metaarray.jl	88.73% <75.00%> (+29.31%)	⬆️
src/methods.jl	95.00% <100.00%> (+3.52%)	⬆️
src/utils.jl	84.61% <0.00%> (-1.10%)	⬇️
src/ranges.jl	93.33% <0.00%> (+13.33%)	⬆️
src/metastruct.jl	100.00% <0.00%> (+37.50%)	⬆️

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