clean up API about 3b-bmfont HOT 2 CLOSED

mostly done, still some missing exports to decide on and other parts that need cleaned up?

to (maybe) export: space-size, char-data,%kerning, v2

other:

• space-size should probably just be calculated once and stored in font object?
• also store preferred fallback character for font once instead of trying 3 things in char-data?
• add-origins should be called from read-bmfont-* instead of read-bmfont, since people might decide to use a specific loader
• possibly factor out a place to do post-loading updates to font data like the above?
• is (chars font) being a hash table part of the API, for things that think calling CHARS per character is too slow? (and is it actually too slow?)

from 3b-bmfont.

In Alloy I use: space-size chars and kernings.

from 3b-bmfont.