jvp_utf8_next() had a fast path for ASCII, but it is not inlined into
its callers, so every byte of a string cost a function call.
Move the end check and the ASCII case into a static inline function in
jv_unicode.h and keep the rest out of line. This speeds up parsing of
string-heavy input by up to 1.17x and string functions such as length,
explode and @html, with the same output for every input.
