(v6.x backport) buffer,util: refactor for performance

[Trott]

internal/util.js defined toInteger() and toLength() but they were only
used by buffer.js. Inlining these small functions results in a small but
statistically-significant performance gain.

PR-URL: #12153
Reviewed-By: Joyee Cheung
Reviewed-By: Michaël Zasso
Reviewed-By: James M Snell

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

[MylesBorins]

are there benchmarks to show this improves perf in v6.x?

[sam-github]

"definied" <--- typo in commit message

[Trott]

are there benchmarks to show this improves perf in v6.x?

@MylesBorins Much to my surprise, this change that had measurable positive performance impact on master appears to have the opposite impact here:

 improvement confidence p.value buffers/buffer-from.js n=2048 len=10 source="arraybuffer-middle" -6.15 % *** 4.991252e-14 buffers/buffer-from.js n=2048 len=10 source="arraybuffer" -7.13 % *** 5.563436e-17 buffers/buffer-from.js n=2048 len=2048 source="arraybuffer-middle" -5.51 % *** 5.312334e-15 buffers/buffer-from.js n=2048 len=2048 source="arraybuffer" -6.82 % *** 4.554484e-14

I haven't looked into why, but for now, I'm going to close this and add a do-not-land-on-v6.x label to the original PR.