[be] emit casts, prettier emit_copy output, fix segfault,
smarter val storage.
* fix segfault in emit_symbol_expr_init. we don't yet properly
reference toplevel variables. workaround, print out "FIXME!" and
create a new pseudo.
* directly inline constants, instead of assigning them stack slots
as pseudos.
* properly emit cast expressions; quite easy with my type-aware
emit_move function. alas, since we assume values will fit into
a machine register, emit_move is [known-]broken for 64-bit ints.
* make asm comments in pseudo-pseudo copy prettier