Revert "Faster floor_sqrt, checked_sqrt"

[tarcieri]

This reverts commit add9780 (#1258).

As reported in #1303, this appears to be miscomputing results.

This reverts until we can track down the cause of the issue.

[tarcieri]

cc @kayabaNerve

[tarcieri]

Looks like there are some public APIs this removed:

  BoxedUint::floor_sqrt_assign_vartime, previously in file /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/crypto-bigint-0.7.4/src/uint/boxed/sqrt.rs:104
  UintRef::carrying_add_assign_mul_limb, previously in file /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/crypto-bigint-0.7.4/src/uint/ref_type/add.rs:100
  UintRef::borrowing_sub_assign_limb, previously in file /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/crypto-bigint-0.7.4/src/uint/ref_type/sub.rs:8

[codecov]

Codecov Report

❌ Patch coverage is 96.63866% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.92%. Comparing base (ec6615f) to head (c715fad).

Files with missing lines	Patch %	Lines
src/uint/boxed/sqrt.rs	92.59%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1304      +/-   ##
==========================================
- Coverage   91.10%   90.92%   -0.19%     
==========================================
  Files         190      189       -1     
  Lines       22731    22493     -238     
==========================================
- Hits        20709    20451     -258     
- Misses       2022     2042      +20     

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