gBASIC for Windows 0.8.0

Full Changelog: v0.7.0...v0.8.0

gBASIC for Windows 0.7.0

The first release with Windows support. gbasic-0.7.0-x64.msix is signed;
gbasic.exe is built statically and needs nothing installed beside it.

One fix changes answers a correct program was already getting. The difference
of two datetimes consulted the machine's timezone, so a day count, an age or a
deadline computed outside UTC could come out a day wrong — in gBASIC's own credit
library that put a sixty-days-past-due loan in the dpd_30 bucket. A datetime
is civil and carries no zone, which is why epoch(dt, zone) takes one; the
subtraction had no business consulting a zone at all. If you have code quoting
an interval, re-check it. Code that ran in UTC was already right.

Also new since 0.6.1

• Projection — sum(rows.amount), and with it mean, min, max, count,
  any and all over a field across a list of records.
• sort on records — sort(rows, { by: ["last", "first"], descending: true }),
  stable, with absences ordering last.
• Set operators — a excluding b, a intersecting b.
• last, slice, present, key, bound.
• Exact integers — arithmetic on whole numbers past 2^53 is exact, and
  9007199254740993 = 9007199254740992.0 is now correctly false.
• mode answers every tied value, or unknown when nothing repeats, instead
  of always returning the first element.
• Tracebacks name the file and owning library of each frame.

Linux builds are published separately at https://gbasic.tedderland.com — see
CHANGELOG.md in the repository for the full entry.

Full Changelog: v0.6.1...v0.7.0