Specialize calls to stream.string_writer.format.

compiler/format_call.m:
    As above.

    Factor out some common code.

compiler/module_imports.m:
    Fix a cut-and-paste bug.

library/builtin_modules.m:
    Add a way to retrieve the name of the stream module, since the compiler
    can now generate calls to stream.put.

tests/hard_codes/opt_format.{m,exp}:
    Expand this test case to include tests of calls to
    stream.string_writer.format.

    Make the existing tests a bit tougher as well.