Store the index and protected index as fixedpoints

[gkaracha]

This is a precursor to addressing #263. It primarily affects the tests, since the switch of the scaling factor from 10^6 to 2^64 increases precision in our calculations and many tight tests had to be adjusted. Using a fixedpoint with such precision is a good middle ground when trying to accommodate arbitrary fractions (such as the ones we expect to get from CFMMs once #263 is implemented) as well as index values with 6 decimal digits of precision (i.e., current state of affairs).

[github-actions[bot]]

Gas costs	2f97225f5b7ae9e5d7315a13563582fe277ccf05	e36197d1b66b06ac38a2c8526340f99fcb52d556	Diff
checker%touch	42373	42390	17
checker%sell_kit	12415	12418	3
checker%remove_liquidity	13265	13268	3
checker%set_burrow_delegate	18300	18302	2
checker%touch_burrow	6100	6102	2
checker%mint_kit	7129	7131	2
checker%deposit_collateral	19981	19983	2
checker%withdraw_collateral	23015	23017	2
checker%update_operators	5587	5589	2
checker%transfer	6309	6311	2
checker%add_liquidity	12570	12572	2
checker%buy_kit	12416	12418	2
checker%deactivate_burrow	23374	23376	2
checker%burn_kit	7358	7359	1
checker%create_burrow	22280	22281	1
checker%activate_burrow	19797	19798	1

Entrypoint sizes	2f97225f5b7ae9e5d7315a13563582fe277ccf05	e36197d1b66b06ac38a2c8526340f99fcb52d556	Diff
receive_price	120	287	167
mark_for_liquidation	16949	17046	97
touch	56749	56697	-52
liquidation_auction_place_bid	2141	2157	16
cancel_liquidation_slice	12130	12122	-8
deactivate_burrow	1465	1459	-6
withdraw_collateral	1298	1292	-6
mint_kit	1537	1531	-6

Test coverage	2f97225f5b7ae9e5d7315a13563582fe277ccf05	e36197d1b66b06ac38a2c8526340f99fcb52d556	Diff
common.ml	97.59	90.36	-7.230000000000004
checker.ml	94.93	94.97	0.03999999999999204
TOTAL	92.47	92.27	-0.20000000000000284