Improved checked_isqrt and isqrt methods

[ChaiTRex]

Improved tests of isqrt and checked_isqrt implementations

• Inputs chosen more thoroughly and systematically.
• Checks that isqrt and checked_isqrt have equivalent results for signed types, either equivalent numerically or equivalent as a panic and a None.
• Checks that isqrt has numerically-equivalent results for unsigned types and their NonZero counterparts.

Added benchmarks for isqrt implementations

Greatly sped up checked_isqrt and isqrt methods

• Uses a lookup table for 8-bit integers and then the Karatsuba square root algorithm for larger integers.
• Includes optimization hints that give the compiler the exact numeric range of results.

Feature tracking issue

isqrt is an unstable feature tracked at #116226.

Benchmarked improvements

Command used to benchmark

./x bench library/core -- int_sqrt

Before

benchmarks:
    num::int_sqrt::i128::isqrt           439591.65/iter  +/- 6652.70
    num::int_sqrt::i16::isqrt              5302.97/iter   +/- 160.93
    num::int_sqrt::i32::isqrt             62999.11/iter  +/- 2022.05
    num::int_sqrt::i64::isqrt            125248.81/iter  +/- 1674.43
    num::int_sqrt::i8::isqrt                123.56/iter     +/- 1.87
    num::int_sqrt::isize::isqrt          125356.56/iter  +/- 1017.03
    num::int_sqrt::non_zero_u128::isqrt  437443.75/iter  +/- 3535.43
    num::int_sqrt::non_zero_u16::isqrt     8604.58/iter    +/- 94.76
    num::int_sqrt::non_zero_u32::isqrt    62933.33/iter   +/- 517.30
    num::int_sqrt::non_zero_u64::isqrt   125076.38/iter +/- 11340.61
    num::int_sqrt::non_zero_u8::isqrt       221.51/iter     +/- 1.58
    num::int_sqrt::non_zero_usize::isqrt 136005.21/iter  +/- 2020.35
    num::int_sqrt::u128::isqrt           439014.55/iter  +/- 3920.45
    num::int_sqrt::u16::isqrt              8575.08/iter   +/- 148.06
    num::int_sqrt::u32::isqrt             63008.89/iter   +/- 803.67
    num::int_sqrt::u64::isqrt            125088.09/iter   +/- 879.29
    num::int_sqrt::u8::isqrt                230.18/iter     +/- 2.04
    num::int_sqrt::usize::isqrt          125237.51/iter  +/- 4747.83

After

benchmarks:
    num::int_sqrt::i128::isqrt           105184.89/iter +/- 1171.38
    num::int_sqrt::i16::isqrt              1910.26/iter   +/- 78.50
    num::int_sqrt::i32::isqrt             34260.34/iter  +/- 960.84
    num::int_sqrt::i64::isqrt             45939.19/iter +/- 2525.65
    num::int_sqrt::i8::isqrt                 22.87/iter    +/- 0.45
    num::int_sqrt::isize::isqrt           45884.17/iter  +/- 595.49
    num::int_sqrt::non_zero_u128::isqrt  106344.27/iter  +/- 780.99
    num::int_sqrt::non_zero_u16::isqrt     2790.19/iter   +/- 53.43
    num::int_sqrt::non_zero_u32::isqrt    33613.99/iter  +/- 362.96
    num::int_sqrt::non_zero_u64::isqrt    46235.42/iter  +/- 429.69
    num::int_sqrt::non_zero_u8::isqrt        31.78/iter    +/- 0.75
    num::int_sqrt::non_zero_usize::isqrt  46208.75/iter  +/- 375.27
    num::int_sqrt::u128::isqrt           106385.94/iter +/- 1649.95
    num::int_sqrt::u16::isqrt              2747.69/iter   +/- 28.72
    num::int_sqrt::u32::isqrt             33627.09/iter  +/- 475.68
    num::int_sqrt::u64::isqrt             46182.29/iter  +/- 311.16
    num::int_sqrt::u8::isqrt                 33.10/iter    +/- 0.30
    num::int_sqrt::usize::isqrt           46165.00/iter  +/- 388.41

Tracking Issue for {u8,i8,...}::isqrt #116226

try-job: test-various