Replace enum fields idents with syms

[cooldome]

During review of #14030 and #14046 I found there is problem with getImpl() for enum types.
Fields are idents and not syms. This makes hard to write typed macros. You have to generate
EnumType.X everywhere instead of X. But it doesn't work for bool enum type. bool.true is not valid.

[Araq]

I'd prefer identToReplace: ptr PNode.

[cooldome]

done

[Araq]

This is a breaking change and needs a migration mechanism.

[cooldome]

old behaviour can now be restored with --oldast switch.

Ready for review

[timotheecour]

good PR but --oldast is really not meaningful at all (eg: how old?) and already had a different meaning, making it hard/impossible for users to migrate their code from right before your PR to right after your PR, since the --oldast would imply other unrelated changes.

The correct way to handle breaking changes is simple and involves both:

• make this controlled by --legacy:enumFieldsIdents (or -d:nimLegacyEnumFieldsIdents if feeling lazy)
• make --useVersion:1.0 (and --useVersion:1.2) define nimLegacyEnumFieldsIdents

For testing legacy, we avoid combinatorial explosion by only testing releases eg --useVersion:1.0 and --useVersion:1.2, and individual breaking changes are best effort.

[Araq]

@timotheecour I'm aware and will change it but was tired of the review ping-pong and so took the shortcut and merged it already.