Add stubs for ssa conversion
Add line break between ssa inst in --test
ssa conversion prints top level string immediates
factor out sourceinfo data in sexpr tokens
parse_stop() to restores FILE after partial parse.

parse() has an internal lookahead buffer of one character that may or
may not be filled depending on what token it encountered last.
parse_stop() sets the underlying FILE to a consistent state and empties
this buffer. This is useful for situations where only some of the FILE
stream is to be consumed by parse() and the rest consumed by some other
means.
Add sexpr_float() and sexpr_int()

This rounds out the sexpr creation functions and makes parse_number() a
little nicer.
Add sexpr_print_nice() to print pretty expressions

This function prints each token as part of an expression, in a format
that would be accepted by the parser. Intended to represent the output
format of later stages where individual token types are not as important.
ssa_convert() returns struct sexpr

Instead of continuing to use it's own data type, ssa also employs a
sexpressions, except they may only use a limited subset of them. This
means less new infrastructure to manage at the cost of increased
potential for inconsistent data due to programming errors in the later
stages.