Pretty-printing support for words that aren't broken relative to their neighbour

[graydon]

Given the way the pretty printer is structured, generating a token at a time, it'd probably be useful to have token types that the pretty printer considered effectively "indivisible" with the word before (or after, or both) them. Otherwise we have to mix stringifying with adjacent tokens.

Examples are "::", "." and ";", as well as possibly the opening and closing parens mentioned in #419. We should never really be breaking after foo in "foo::bar" or "foo.bar" or "foo;" but we do sometimes.

[msullivan]

Graydon says this is probably fixed.