More math char classes (fix #159)

[nyuichi]

I added five math char classes

• sans serif
• bold serif
• italic sans serif
• bold italic sans serif
• typewriter

and three math font selectors

• \mathit
• \mathsf
• \mathtt.

I also added test-typefaces.saty to the tests directory. I did not add the compiled results to the repository but instead put them here for reviewing.

test-typefaces.pdf

[nyuichi]

rebased