Add support for more forms of format specifications

[udoprog]

This adds support for more formatting specifications. In principle almost everything should parse and be communicated to the internal FormatSpec type, but not everything is implemented yet.

However, the following things are implemented for strings and numbers:

• fill character
• alignment <, ^, >.
• zero-aware filling (for numbers only)
• display and debug formatting
• hex (upper and lower) and binary formatting (integers only)
• positive and negative sign spec (numbers only)

I'll added more test cases here:
https://github.com/rune-rs/rune/pull/144/files#diff-1a0bb1364e0488bce0d80ecf20950d8e

And more to follow.