Move emission of the `Ret` instruction into `vmgen.genExpr` and have the
`genExpr` procedure in `vmjit` emit an 'eof' instruction. This brings
the shape of the latter closer to that of `genStmt`.

The state and required contextual data is moved into the new
`CodeGenCtx` type owned by `vmgen`. All types only referenced by the
now-moved code are moved with it.

The field doesn't exist anymore.

Co-authored-by: Saem Ghani <saemghani+github@gmail.com>