Parser: release temporary compilation memory

Parser: release temporary compilation memory

Create a pool for each parser invocation and allocate parser nodes, array-item
storage, continuations, preread tokens, and active labels from it.  Destroy the
pool after generation and on parser or generator failures.

Keep atoms, lambdas, regexp patterns, scopes, variables, bytecode, and runtime
constants in the VM pool.  Clear persistent scope AST roots before releasing
the temporary pool.