numerical precision fixes for long calcs

[jrhea]

This PR fixes several rounding issues that were uncovered in tests and adds mulUp and divUp to the FixedPointMathLib. The following issues have been solved:

1. test_open_long_with_small_amount() with a trade amount of 0.01e18 had an issue where the average maturity time was unable to pass the following check:

        assertApproxEqAbs(
            poolInfoAfter.longAverageMaturityTime,
            maturityTime,
            1
        );

this issue was fixed in _calculateAverageMaturityTime() by using divUp() instead of divDown()

2. test_close_long_redeem() had an issue where bob ends up with more base than bonds after redemption. This issue was fixed in HyperDriveMathLib's calculateInGivenOut() by ensuring the YieldSpace math isn't called on redemption.

3. test_open_long() had an issue where a small long position would make the APR go up instead of down. The root cause was a rounding bug in YieldSpaceMath when we would calculate 1/(1-tau). This was fixed by using divUp() instead of divDown(). I mapped out the difference in using divUp() and divDown() with various trade sizes. Here is a summary of what I found:

        trade size: 10e18
        with divDown: 49999999999886067
        with divUp: 49999996900228853
        delta: 49999999999886067 - 49999996900228853 = 3099657200
        trade size: 1e18
        with divDown: 50000006551536041
        with divUp: 49999975554963707
        delta: 50000006551536041 - 49999975554963707 = 30996572304
        trade size: 0.1e18
        with divDown: 50000057980327600
        with divUp: 49999762774876600
        delta: 50000057980327600 - 49999762774876600 = 295205451000
        trade size: 0.01e18
        with divDown: 50000583700909500
        with divUp: 49997631646399300
        delta: 50000583700909500 - 49997631646399300 = 2952054510208

[coveralls]

Pull Request Test Coverage Report for Build 4198542259

• 0 of 6 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 26.863%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
contracts/libraries/FixedPointMath.sol	0	3	0.0%
contracts/libraries/HyperdriveMath.sol	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
contracts/libraries/HyperdriveMath.sol	1	0%

Totals
Change from base Build 4196898498:	-0.2%
Covered Lines:	137
Relevant Lines:	510

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💛 - Coveralls

[jalextowle]

Left a few nits, but good work sir 🖖