Rewrite – Better Unicode support, better performance, better whitespace support, more macros.

The awful old lexer which did not handle unicode correctly, and did not handle whitespace sensibly... is gone!

Also, the old code's complete lack of good memory management (with clones everywhere) is rewritten the Rust way, with carful consideration of lifetimes, ownership, borrowing, &c.

We also get some more macros, notably

• (%format ...) for Rust-style string formatting.
• (%os/env ...) for accessing environment variables.

With lots more low-hanging fruit to come. :)