Improve token stream pretty printing.

[nnethercote]

Specifically:

• No space before or after ::, e.g. a::b instead of a :: b.
• No space between ! and (, e.g. foo!() instead of foo! ().
• No space between ! and [, e.g. #![...] instead of #! [...].
• No space before :, e.g. a: u32 instead of a : u32.
• No space before ;, e.g. struct A; instead of struct A ;.

r? @petrochenkov