This adds parsing for expressions like 'x as Ty[0]' which will
immediately error out, but still give the rest of the parser a valid
parse tree to continue.

This is almost entirely refactoring and message changing, with the
single behavioral change of panicking for unexpected output.

Previously this just errored out on all usages of type ascription,
which isn't helpful.

This is a modified version of estebank's suggestion, with a bit of
extra cleanup now that we don't need the different cases for if we can
turn a span into a string or not.

Implementes suggeseted changes by Centril.

This checks whether the memory location of the cast remains the same
after atttempting to parse a postfix operator after a cast has been
parsed. If the address is not the same, an illegal postfix operator
was parsed.

Previously the code generated a hash of the pointer, which was overly
complex and inefficent. Casting the pointers and comparing them
is simpler and more effcient.