Add more varying testing for most parsing functions.
Tests emphasize whitespace, literals, timezones, and timezone
delimiters (colons and whitespace).

Add tests for multiple-byte characters and combining characters
in and around data and parsing formats.

These tests are added to aid humans verifying the next commit that
changes parsing behavior.

Issue chronotope#660

Be exact about allowed whitespace around and between data and
parsing formats for all parsing.
Except RFC 2822 which explicitly allows arbitrary whitespace.

Issue chronotope#660

Add tests for timezone parsing infinite whitespace and colons.

Prepatory tests for next commit around constraining timezone and
colons.

Constrain timezone middle-colon separator string from
infinite intermixed whitespace and colons
to possible patterns `":"`, `" "`, `" :"`, `": "`, or `" : "`.
A reasonable trade-off of previous extreme flexibility for
a little flexbility and concise input.

Issue chronotope#660