Update C-String literals to reject NUL

[nnethercote]

Which moved the checking for NUL chars in C string literals earlier.

See rust-lang/rust#119172

[nnethercote]

This was the least worse way I could see to do this, but I'm sure there are other possibilities.

[mattheww]

Suggested change

	>       | BYTE_ESCAPE _except `\0` or `\x00`\
	>       | UNICODE_ESCAPE _except `\u{0}`, `\u{00}`, …, `\u{000000}`\
	> &nbsp;&nbsp; &nbsp;&nbsp; | BYTE_ESCAPE <sub>_Except `\0` or `\x00`_</sub>\
	> &nbsp;&nbsp; &nbsp;&nbsp; | UNICODE_ESCAPE <sub>_Except `\u{0}`, `\u{00}`, …, `\u{000000}`_</sub>\

The italics weren't coming out properly. I've imitated the markup under "Reserved prefixes", which I think is the most similar case.

[nnethercote]

I had omitted the trailing _ to close the italics. I've added that instead of using <sub>, see what you think?

[mattheww]

I have no objection to that

[ehuss]

Thanks!

[ehuss]

ZeroByte doesn't seem to be defined anywhere, and I think could be confused with DIGIT ZERO. Could we use a different term here? Perhaps just Null?

Suggested change

	>       ~\[`"` `\` _IsolatedCR_ _ZeroByte_]\
	>       ~\[`"` `\` _IsolatedCR_ _Null_]\

[nnethercote]

I didn't define ZeroByte because IsolatedCR also isn't defined. I have changed ZeroByte to Nul. (Not Null, because that tends to be used for the zero pointer while Nul tends to be used for the zero character.)

[mattheww]

For what it's worth, the IsolatedCR definition is hiding in the "Comments" chapter.

[nnethercote]

I've changed it from "Nul" to "NUL", because "NUL" is defined earlier in the sentence "The null escape is the character U+0030 (0) and denotes the Unicode value U+0000 (NUL)." which I hope is good enough?

[ehuss]

Sounds good. I don't know if the world has an agreement if it is NULL or NUL. They seem to be used interchangeably, and I don't think there is any ambiguity what either spellings mean here, which means I think it is largely a matter of personal preference.

[nnethercote]

Thinking some more, I suspect "null" is the proper word, and NUL is an ASCII-ism (because the control chars all have two or three letter abbreviations) that has crept into ordinary use, including Unicode names.

[ehuss]

Thanks!