Also, updates some of the mzlib files to point at `racket/*' libraries
rather than to `scheme/*' ones.
original commit: ac26fe75546b5182d78f18c2cd882f0f440849e2
so that `@racketmodname[lazy], for example, typesets correctly
when `lazy' is imported for-label
original commit: c8999c25413352de46cd1f8006a768ff74938163
- introduce 'code-inset style for nested blocks
- use style instead of spaces for code indentation
- sigplanconf adds vertical space around 'code-inset
- more ...0 variants to compose better
original commit: bbc5533938b1dac8f14005210fa9fe552dfa7e8c
