Asm Undef handler scraps return value from C handler

[jonathanpallant]

We tell people they can return a value from a handler and then we throw the value away.

Remove the push/pop LR from _asm_default_undef_handler, and write a test for resuming the instruction after.

[jonathanpallant]

See #30

[jonathanpallant]

Fixed in #30