Modernize core::extfmt::ct

[ghost]

This is effectively a modernization of the parsing portion of format string handling (i.e., core::extfmt::ct).

It contains the following changes:

• The structural record Conv is replaced with an otherwise identical struct Conv.
• The structural records returned from the parse_* functions are replaced with instances of struct Parsed<T> { val: T, next: uint }.
• All types have a derived cmp::Eq implementation to facilitate simpler test code.
• The parse_* and peek_num functions are rewritten to remove unnecessary moves and copies and to use simplified logic.
• parse_fmt_string is rewritten to use two byte indices instead of string concatenation to build up each string piece.
• parse_type now correctly handles multibyte characters; although all current types are represented by single-byte characters, this prevents the function from failing through invalid indices passed to substr and allows any unknown type specifier character to be printed in its entirety.
• Most functions now have associated unit tests.
• The module is formatted more consistently.

Note that all function signatures remain the same modulo the removal of the structural records, so these changes should not affect the code generation or runtime aspects of formatting.

Feedback is welcome.

[catamorphism]

I'll take a look at this!

[catamorphism]

Great cleanup work, thanks for this!