Implement most of the builtin time module

[unexge]

Partly addresses #72.

With this PR we still fail to pass some OPA tests:

$ cargo test --test opa --features opa-testutil -- time # You need to pass `opa-testutil` for `test.sleep` function
...
OPA TESTSUITE STATUS
    FOLDER                                    PASS FAIL
    time                                    :   28    4

    TOTAL                                   :   28    4

MISSING FUNCTIONS
          FUNCTION                                 FAILURES
       1: time.parse_duration_ns                   1
          TOTAL                                    1

They fail because we can't parse Go's duration (1m32s) and time layout (2006-01-02T15:04:05.999999999Z07:00) at the moment.
I plan to address them in a follow-up PR.

[anakrish]

LGTM.

We'd need to include licenses as appropriate.

[anakrish]

This function can be moved to a separate file that lists the original license.

[unexge]

I tried to address it with 56b3c72, but I am not sure if that's sufficient

[anakrish]

I believe that is good for now.

Later, we might have to declare a THIRD_PARTY_NOTICES file like https://github.com/openenclave/openenclave/blob/master/THIRD_PARTY_NOTICES
or find a way to avoid the code.