Improving the performance of validate_ascii

[lemire]

Short title (summary):

The validate_ascii function is made faster for most kernels

Description

@ChALkeR first raised the issue in nodejs/node#61122

Both validate_ascii and validate_ascii_with_errors are very fast, but when the content is pure ascii, validate_ascii should be faster. This PR fixes the issue.

Type of change

• Bug fix
• Optimization
• New feature
• Refactor / cleanup
• Documentation / tests
• Other (please describe):

[ChALkeR]

There are also some very weird cases I noticed (not blocking this PR)
Will check later and file a separate issue (or DM me in Slack and I'll demonstrate the weirdness, but I'm not yet sure where it comes from)

[lemire]

There are also some very weird cases I noticed (not blocking this PR)

If you have found issues, my preference is that you open a public issue as we are an open community project.

But if you have security issues, I can be reached privately easily by email or you can reach @pauldreik.

[ChALkeR]

Atm it's just in the "timing seems very strange and needs investigation" stage, filing an issue with that is too early - it lacks data for that yet

It's not a security issue

Given that preference, I'll file this sometime later when I'll get more understanding on what's happening in that specific case.