The logic used by Date.fromString and Date.parse has a few issues:

• Incorrect handling of fractional seconds having more than 3 decimal places. For example, "2000-01-01T00:00:00.123456789Z" was being treated as having 123456789 milliseconds, making the result wildly too large. Such values should be truncated to 3 decimal places before being interpreted as milliseconds.
• Incorrect handling of fractional seconds having less than 3 decimal places. For example "2000-01-01T00:00:00.1" was treated as having 1 millisecond, though it should be one tenth of a second (or 100 milliseconds).
• Ignoring any UTC offset (such as +05:30 or -08:00), which is required by ISO 8601, RFC 3339, and the ECMAScript standard date format.

This PR addresses all three issues, and updates the existing Date#fromString test.

I also made two small unrelated changes:

• Replaced usages of I32.parseInt with i32.parse in std/assembly/date.ts to resolve deprecation warnings
• Regenerated the fixture for std/math test to resolve "fixture mismatch" error that was occurring on a clean clone of the repo before I made any changes. (Not sure what update caused that, but compiler tests were failing.). (reverted this due to CI test failure. See comments below.)

Thanks.

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