Fix compiler abort looking for stream.put (Mantis bug 369).

compiler/module_imports.m:
    Implicitly import stream.m if the module being compiled may call
    stream.string_writer.format.

compiler/simplify_proc.m:
    Keep stream.put around until format_call.m may need it.

compiler/pred_table.m:
    If looking for a predicate fails, generate an error message that mentions
    the module name part of the fully qualified predicate name being
    looked for.

tests/hard_coded/stream_put_bug.{m,exp}:
    New test case, based on one contributed by sebgod (the filer of
    Mantis bug 369).

tests/hard_coded/Mmakefile:
    Enable the new test case.
Minor improvements.

compiler/simplify_goal_ite.m:
    Fix the English in some comments.

compiler/simplify_goal_switch.m:
    Give a variable a better name.