Add bash comparison tests
Add X{a..#}X test case
Alpha sequence is only letters, not all non-numbers
Do not append last item in sequences where the step passes by the end
Do not include \ in alpha sequence
Pad negative numbers after the -, not before

Ie, {-002..0} is -001 0000 0001, not 00-2 00-1 0000
Add {-10..00} test case
Add X{{a,b},}X test case
Allow negative inc's in alpha sequences
Elide out empty results, like Bash does
Add {{a,b}c} test case
Add {a,\{a,b}c} test case
Support \{ properly
More efficient \{ handling
Refactor part-parsing.  All but 2 tests passing.

This refactors a lot of things, but was tricky to do atomically.

The first part is moving the comma-delimited parsing to a separate
function, along with several other things that were being done
inline unnecessarily.

The next is to avoid a bunch of excessive parsing of range parts,
and the prefix which comes before any matched open-{.
2 more dollar tests

Accidentally made these fail while trying out a strategy to make
escaping more effective.
Escape braces and commas more thoroughly
Escape periods as well as other chars
Allow escaping of slashes, before other things
Handle {a},b} as {a\},b} -> a} b

Now passing all tests.