Switch Clock to BigInt and fix monotonicity

[albe]

Due to reduction of accuracy of the hrtime based clock to microseconds, the clock was no longer monotonic.

This fixes that, makes use of hrtime.bigint() (see #72 (comment)) and removes the decimal-encoded clock accuracy, which was not preserved for high clock ticks anyway (only up to ~30 years, which is likely still sufficient, but not guaranteed).

Also, this adds tests for the Clock to verify monotonicity and that it can cover at minimum 100 years (from it's epoch into the future).

[coveralls]

Coverage increased (+0.05%) to 97.956% when pulling d94591e on clock-fixes into 401c9d2 on main.