Use modern enums in compiler

[cooldome]

Reduce number of constant strings, but reusing string representation of nim enums that compiler will generate anyway.
I managed to shave off 35 kb from the compiler binary with this trick.

I haven't noticed a slowdown in compiler despite I know about reprEnum string allocation overhead.
Possibly we need to improve codegen for reprEnum before making this change.

Also I think we need to get rid of findStr proc in wordrecg in favour of parseEnum. It is efficient now

[Clyybber]

I did similar things in #14777 (comment), see Araqs comment for why I reverted those changes in that PR.

[timotheecour]

That's now 3 people (i had also attempted a similar thing some time ago) that have tried to replace those array[Enum, string]. Unfortunately we're going in circles. See #14777 (comment)

I don't like the enumValue = "string repr" pattern

Can we then reconsider #14008 ?

Yeah, that's a nice compromise. Bring it on please!

but then it got closed (along with its fusion alternative).

[Araq]

That's now 3 people

And that's my threshold so I'm willing to accept this solution now.

[cooldome]

Looks good now, I will look into speeding up reprEnum in future PRs

[timotheecour]

unfortunately this still leaves CallingConvToStr: array[TCallingConvention, string] = ["N_NIMCALL",... in compiler/ccgtypes.nim but without
https://github.com/nim-lang/Nim/pull/14008/files#diff-6b7c28355957ab974fa38584bc810d1aac557bdcc7874e98d8ec005cc2200d72 that's the best one can do; still a good improvement.