codegen: don't use PSym for labels

[zerbina]

Summary

In the code generators, represent block labels with a dedicated type
and
node, instead of using PSym for that. This is an internal-only
change,
and another step towards removing all PSym usage from the code
generators.

Details

• add the cnkLabel node kind to CgNode
• it stores a BlockId , and appears as sub-node for both
  cnkBlockStmt
  and cnkBreakStmt
• a BlockId is not a unique ID within a Body -- instead it
  identifies the enclosing block for a break. This matches how the
  code generators previously used skLabel symbols
• cgirgen now no longer queries or modifies the identifier cache
  (IdentCache), so all usages of the type are removed. The idents
  import is turned into a narrow import

The code generators are updated accordingly:

• cgen uses its blocks stack not only for cnkBlockStmts, but for
  C blocks ({}) in general. Therefore, TBlock stores the BlockId
  offset by 1, leaving '0' to mean "not corresponding to a
  cnkBlockStmt"
• jsgen uses the BlockId as an index into the blocks stack, which
  is now a seq[int] (the previously used TBlock type had only a
  single field)
• vmgen also uses the BlockId as an index into its blocks stack,
  making the previous search logic obsolete. The withBlock and
  popBlocks routines were only used in a single place -- they're
  inlined and removed. In addition, the blocks stack embeds the
  ending list directly, removing the need for the TBlock type

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• the next step in removing PSym from the code generators
• made possible by transf: lower all unlabeled break statements #918