print more bs instrs with test_compilerLib
add elab_top and infer_top

might be buggy; also elab_prog and infer_prog look buggy...
add parse_top and lex_next_top (replace lex_until_toplevel_semicolon)

parse_top is a hack at the moment, and should be replaced by something
nicer.
hacky bug fix to lex_next_top

magnus will write a correct definition later
rework repl_fun to take tops one at a time

This change is primarily motivated by making rollback in the case of an
unhandled exception easier, because we can now commit the changes after
each succesfully evaluated declaration/module. I think it also makes the
intended semantics easier to understand. (The repl semantics is not
updated in this commit though.)
fix compiler for new repl API

it currently doesn't try to handle exceptions but simply indicates
blindly that the result of evaluation was successful.