Close#1885, #2140, #2011, #2220, #2485, #2073
Rename the FORK_OPT opcode to TRY_BEGIN, add a TRY_END opcode, and wrap
errors when raising through a TRY_END so that they will not be caught by
the matching TRY_BEGIN.
Now a `try exp catch handler` expression generates code like:
TRY_BEGIN handler
<exp>
TRY_END
JUMP past_handler
handler: <handler>
past_handler:
...
On backtrack through TRY_BEGIN it just backtracks.
If anything past the whole thing raises when <exp> produced a value,
then the TRY_END will catch the error, wrap it in another, and
backtrack. The TRY_BEGIN will see a wrapped error and then it will
unwrap and re-raise the error.
If <exp> raises, then TRY_BEGIN will catch the error and jump to the
handler, but the TRY_BEGIN will not stack_save() in that case, so on
raise/backtrack the TRY_BEGIN will not execute again (nor will the
TRY_END).
Nico Williams
1cf6515c06