make var, let work in static, const sections

[timotheecour]

refs nim-lang/RFCs#276

see tests tests/misc/trfc_276.nim

for static: this PR should fix existing issues

• fix [vm] Set/string/seq inside loop not initialized properly #10938
• fix Top level variables inside loops are not reset in VM #13312
• fix opcLdDeref (eg via addr) gives wrong results in inside static block #13887

for const: this PR should improve existing issues

• refs Const block variable memory being reused unexpectedly #13918
• refs Error in const block inside proc #12172
• refs Consts that call templates cannot evaluate whilst in procedure bodies #13795 [1]

note 1

this PR uses the genPNode introduced in #17938

[1]

import std / sequtils
type SomeEnum = enum
  Foo = "foo", Bar, Baz
proc someProc() =
  static:
    let y1 = SomeEnum.toSeq # now works
  const y2 = SomeEnum.toSeq # doesn't work, it'd require to make `needConstWrap` smarter in this PR

likewise, #14645 works with:

      static:
        let rx = re(&"")

but not yet with const rx = re(&"") for same reason

note 2

I'm not sure how to get rid of needConstWrap (only needed for const, not static); without it some tests fail, see the test with: const SymChars: set[char] = {'a' .. 'b'}

note 3

still fails with some tests...
see #18637 for an alternative that doesn't use intermediate procs, it almost works but has some issues; ... it's hard

[stale]

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