update: restricting parameter names to not collide with ones we use for OpenQASM generation.

[krneta]

Issue #, if available:
#603

Description of changes:
When generating OpenQASM we decided to use 'b' and 'q' for 'bits' and 'qubits' variables, but this can collide with parameters chosen by customers. I've renamed these to __bits__ and __qubits__ and have changed our requirements for FreeParameter names so that we exclude names that start with double underlines. I've also updated the verification function for our FreeParameter names to be closer to the identifiers definition from OpenQASM to further reduce changes there will be transformation errors (some of which are mentioned in the ticket above).

This check is not exact, so it's possible that we'll allow for FreeParameter names that are not translatable to OpenQASM, since there's a trade-off between performance and upkeep (and keeping track of every reserved word in OpenQASM).

Testing done:

• Ran tests and unit tests and did searches. I hope there's nowhere that I've missed a 'b' or 'q' used in this way because they're quite hard to search for.

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[codecov]

Codecov Report

Merging #675 (7e8d3f0) into main (885a63b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #675   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files          122       122           
  Lines         8133      8144   +11     
  Branches      1822      1826    +4     
=========================================
+ Hits          8133      8144   +11     

Files Changed	Coverage Δ
src/braket/circuits/serialization.py	100.00% <ø> (ø)
src/braket/pulse/ast/qasm_transformer.py	100.00% <ø> (ø)
src/braket/circuits/circuit.py	100.00% <100.00%> (ø)
src/braket/parametric/free_parameter.py	100.00% <100.00%> (ø)