Add fuzzing for calendar arithmetic code

[Manishearth]

This also updates the constructor fuzzer with the latest code. I guess it doesn't get CI'd.

I split out the "construct a date" code into common.rs so it can be reused.

These fuzzers appear to be running smoothly. I had to cap day values for the add fuzzer within Temporal range, though I think in the long run ICU4X should be throwing out of range errors for sufficiently large duration values.

[Manishearth]

add fails (twice)

thread '<unnamed>' (2739167) panicked at /home/manishearth/dev/icu4x/components/calendar/src/duration.rs:244:21:
0 + DateDuration { is_negative: false, years: 4294901760, months: 256, weeks: 0, days: 0 } out of year range
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
==2739167== ERROR: libFuzzer: deadly signal

Failing input:

	fuzz/artifacts/add/crash-dc307a0329c2b1ed8f4ac344f7cedb7b04e90ce5

Output of `std::fmt::Debug`:

	FuzzInput {
	    ymd: Ymd {
	        year: 0,
	        month: 0,
	        day: 0,
	        month_interpretation: Ordinal,
	    },
	    duration: DateDuration {
	        is_negative: false,
	        years: 4294901760,
	        months: 256,
	        weeks: 0,
	        days: 0,
	    },
	    overflow_constrain: false,
	    cal: Dangi,
	}

Reproduce with:

	cargo fuzz run add fuzz/artifacts/add/crash-dc307a0329c2b1ed8f4ac344f7cedb7b04e90ce5

Minimize test case with:

	cargo fuzz tmin add fuzz/artifacts/add/crash-dc307a0329c2b1ed8f4ac344f7cedb7b04e90ce5


────────────────────────────────────────────────────────────────────────────────

Failing input:

	fuzz/artifacts/add/slow-unit-430c4ed1b80e79f55b6c7e9bbaf97ffebd93d326

Output of `std::fmt::Debug`:

	FuzzInput {
	    ymd: Ymd {
	        year: 0,
	        month: 0,
	        day: 0,
	        month_interpretation: Ordinal,
	    },
	    duration: DateDuration {
	        is_negative: false,
	        years: 0,
	        months: 1281,
	        weeks: 0,
	        days: 8589934592,
	    },
	    overflow_constrain: false,
	    cal: Dangi,
	}

Reproduce with:

	cargo fuzz run add fuzz/artifacts/add/slow-unit-430c4ed1b80e79f55b6c7e9bbaf97ffebd93d326

Minimize test case with:

	cargo fuzz tmin add fuzz/artifacts/add/slow-unit-430c4ed1b80e79f55b6c7e9bbaf97ffebd93d326

[Manishearth]

Temporal.PlainDate.from({year:0, month: 1, day: 1, calendar: "dangi"}, {overflow: "constrain"}).add({years: 4294901760, months: 256})

crashes v8

Temporal.PlainDate.from({year:0, month: 1, day: 1, calendar: "dangi"}, {overflow: "constrain"}).add({months: 1281, days: 8589934592})

takes forever to run but eventually fails without crashing

[Manishearth]

I've done the same thing I did in my original PR: constrained the fuzzer appropriately to match our client usage (which I'm changing in boa-dev/temporal#615).

As we start to fix these issues we can relax the fuzzer. This keeps the fuzzer useful: I am able to fuzz what is (or will be) deployed in client use cases, while we can always locally tweak it as our confidence range expands.