Encoded bytes are moved from the end to the front so each byte is
only moved once.

Encoding is somewhat rearranged to operate on a slice into the
output buffer. This makes it easier to avoid clobbering any
existing bytes in the buffer, as well as paving the way to slice-
based encoding needed for a Display wrapper, stream adapters, etc.

At the cost of a 30% encode speed hit, the only unsafe used outside
of line wrapping is to view the output buffer as a Vec<u8>.

Hand unroll the main encode loop 4x. 8x was barely better on longer
inputs, but only barely, and it hurt shorter inputs that then couldn't
enter the loop at all.

The old tests that exhaustively check strings a couple bytes long
weren't that useful, and only checked one config. Using the random
config helper in src/tests.rs is a better use of wall clock time
when waiting for tests to run.