This basically does what is necessary to get everything compiled and
tests passing.
We'll add on time zone and offset stuff in a subsequent commit.
Note that we now depend on `icu_calendar` and `icu_time` directly, with
the latter being optional (but enabled by default). In particular, one
can do useful things with just conversions to dates with `icu_calendar`.
But I expect most folks will want both.
I'm not sure when or how exactly it happened, but in the last weeks,
I've noticed that `rustc` gets effectively stun-locked whenever I make a
change to a source file in Jiff. A quick examination of what the fuck my
computer is doing seems to reveal that it's spending oodles of time
compiling diesel over and over.
I have no idea why this is happening and I don't really care to spend
the time unraveling the mysteries of diesel.
So I took a hammer to the problem. I have effectively shunted all
examples and all "integration" crates out of Jiff's core workspace and
into their own little bloated fiefdoms. To compensate for the fact that
`cargo test --all` no longer tests these things, I've added shell
scripts to run the requisite tests. And those shell scripts are now run
in CI.
I'm now back to a state where I can save a file in Jiff and I get
sub-second `cargo check` response times.
