Change ParamResolver keys to be strings

[dstrain115]

• This PR changes ParamResolvers to only use string keys.
• Previously, ParamResolver keys could be strings or symbols.
• If a symbol is detected, the ParamResolver will remake the dictionary using only strings.
• This simplifies the logic and improves performance since sympy Symbols can be slower to create and hash.

Performance implications:

• Creation speed is unchanged unless you have symbols as keys, then creation takes about twice as long. (For 1000 parameters, this is a change from about 0.6ms to 1.3ms).
• Lookup speed is now about ~120-130ns
• Before it was: -- ~200ns for string lookup with string keys
  -- ~250ns for symbol lookup with string keys
  -- ~850ns for string lookup with symbol keys
  -- ~350ns for symbol lookup with symbol keys

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.38%. Comparing base (036f969) to head (ed31386).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7714   +/-   ##
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  Coverage   99.38%   99.38%           
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  Files        1091     1091           
  Lines       97813    97820    +7     
=======================================
+ Hits        97212    97219    +7     
  Misses        601      601           

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