py/parse: Simplify handling of errors by raising them directly.

The parser was originally written to work without raising any exceptions
and instead return an error value to the caller.  But it's now required
that a call to the parser be wrapped in an nlr handler, so we may as well
make use of that fact and simplify the parser so that it doesn't need to
keep track of any memory errors that it had.  The parser anyway explicitly
raises an exception at the end if there was an error.

This patch simplifies the parser by letting the underlying memory
allocation functions raise an exception if they fail to allocate any
memory.  And if there is an error parsing the "<id> = const(<val>)" pattern
then that also raises an exception right away instead of trying to recover
gracefully and then raise.