Make InlineStrings C-compatible

[KristofferC]

Right now an InlineString is not C-compatible because there is no null trailing byte (in the case of all characters occupying the string) because the last byte stores the length of the string. There is a trick where, instead of storing the length of the string in the last byte, you store "the space left in the string", so if the max chars is 31 and you have 10 characters, you store 31 - 10 = 21 in the last byte. The point being that in the case of the string being full you would store a 0 there which would double-dip as the capacity left (which is zero) AND the trailing null byte making the string C-compatible.

This would allow wrapping an InlineString in a Ref, taking a pointer and passing it to a standard C-string function. And Julia will likely optimize away the Ref allocation.

[caseykneale]

This is a little tangential to the original issue - but the lack of NULL byte handling can lead to somewhat unexpected errors. Granted, not in main-stream use cases, but, here's an example:

Symbol(InlineString127("coookies\0"))
ERROR: LoadError: ArgumentError: Symbol name may not contain \0

Respecting the audience for this package this probably isn't a problem, but it may be worth pointing out.

[bkamins]

This is the same as with normal String:

julia> Symbol("coookies\0")
ERROR: ArgumentError: Symbol name may not contain \0