Revert BuiltinFnName

[odashi]

Overview

This change reverts BuiltinFnName enum.

CC: @ZibingZhang

Details

BuiltinFnName was introduced by #130 to constrain keys in BUILTIN_FUNCS. I originally thought that it was useful, but after several development I realized that using raw string is more desirable:

• Enum introduces a strong type constraint. If I introduced mypy in my local environment, BUILTIN_FUNCS rejects all strings that are not defined in the BuiltinFnName, which is not a desirable behavior.
• We are required to write very long ientifiers, e.g., constants.BuiltInFnName.EXP.value rather than "exp". The identifier also involves the same string EXP with the function name, meaning that we eventually need to write the function name with longer prefix every time.
• The enum is rarely used: in many cases the current implementation keeps the string representation.

References

• NA

Blocked by

• NA

[ZibingZhang]

LGTM

[ShigekiKarita]

Nice simplification.