Spearbit #56: Safely cast from uint256 to int128 or int256

[jalextowle]

Addresses Spearbit Issue #56.

[github-actions]

Hyperdrive Gas Benchmark

Benchmark suite	Current: 8a5f491	Previous: 46c834d	Deviation	Status
addLiquidity: min	33893 gas	1546 gas	2092.3027%	🚨
addLiquidity: avg	147650 gas	67936 gas	117.3369%	🚨
addLiquidity: max	405338 gas	293159 gas	38.2656%	🚨
checkpoint: min	29254 gas	1182 gas	2374.9577%	🚨
checkpoint: avg	107849 gas	48477 gas	122.4746%	🚨
checkpoint: max	211883 gas	191559 gas	10.6098%	🚨
closeLong: min	31450 gas	1492 gas	2007.9088%	🚨
closeLong: avg	100614 gas	29374 gas	242.5274%	🚨
closeLong: max	228204 gas	152303 gas	49.8355%	🚨
closeShort: min	31327 gas	1494 gas	1996.8541%	🚨
closeShort: avg	100162 gas	33444 gas	199.4917%	🚨
closeShort: max	198761 gas	149156 gas	33.2571%	🚨
initialize: min	31283 gas	1451 gas	2055.9614%	🚨
initialize: avg	249251 gas	213962 gas	16.4931%	🚨
initialize: max	320571 gas	253953 gas	26.2324%	🚨
openLong: min	33503 gas	1499 gas	2135.0233%	🚨
openLong: avg	130146 gas	51658 gas	151.9377%	🚨
openLong: max	251992 gas	185562 gas	35.7994%	🚨
openShort: min	33870 gas	1520 gas	2128.2895%	🚨
openShort: avg	138914 gas	51482 gas	169.8302%	🚨
openShort: max	365088 gas	181325 gas	101.3445%	🚨
redeemWithdrawalShares: min	31205 gas	1488 gas	1997.1102%	🚨
redeemWithdrawalShares: avg	72269 gas	22375 gas	222.9899%	🚨
redeemWithdrawalShares: max	150959 gas	109301 gas	38.1131%	🚨
removeLiquidity: min	31213 gas	1530 gas	1940.0654%	🚨
removeLiquidity: avg	222669 gas	151225 gas	47.2435%	🚨
removeLiquidity: max	369048 gas	325781 gas	13.2810%	🚨

This comment was automatically generated by workflow using github-action-benchmark.

[jrhea]

What about line 194 and 233 in FixedPointMath?