rename case statement macro from match to case

[metagn]

closes nim-lang/RFCs#332

Mostly to see what breaks

[metagn]

CI seems fine. Maybe don't merge yet, PRs could be made to fusion etc in anticipation of this first, also could lift from experimental as said in the RFC but PRs for that would have to be after the merge

[timotheecour]

• @hlaaftana after running git bisect, this seems to cause a regression, fusion/matching now doesn't work:

when true:
  import fusion/matching
  {.experimental: "caseStmtMacros".}
  case (1, 2):
    of (3, 4), (1, 2):
      discard
    else:
      discard

works before this PR, fails after

Error: attempting to call undeclared routine: 'case'
    case (1, 2):

• I also notice we don't even have fusion in important_packages; we should add it; having it would've prevented this regression. Note that fusion CI is green but would be red the next time it runs

[metagn]

after running git bisect, this seems to cause a regression, fusion/matching now doesn't work:

The change to rename match to `case` in fusion is in nim-lang/fusion#61, so yes fusion HEAD is broken for now. The change could be made in a separate PR but that PR seemed like it was about to be merged so I commented on it.

[timotheecour]

• but Pattern matching follow-up PR. fusion#61 does a whole lot of other things, why not have a hot-fix that just renames it to unbreak fusion/matching?
• also, how about adding nimHasRenamedMatchCase to condsyms.nim so that you can write code that works both before and after this change? (eg, so that 1.4.2 can write caseStmtMacros too)

[metagn]

• but nim-lang/fusion#61 does a whole lot of other things, why not have a hot-fix that just renames it to unbreak fusion/matching?

• also, how about adding nimHasRenamedMatchCase to condsyms.nim so that you can write code that works both before and after this change? (eg, so that 1.4.2 can write caseStmtMacros too)

Would be fine with both of these getting done, just initially neither seemed necessary to me.