Improve performance of UTF-8 validation (#3675)

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.

tests/jq.test

@@ -84,6 +84,11 @@ null
 "Zm/Ds2Jhcgo="
 "foóbar\n"
 
+# Invalid UTF-8 next to ASCII runs is still replaced
+map(@base64d)
+["YWJjZGVmZ2hpav9rbG1ub3A=", "YWJjZGVmZ2hpasOpa2xtbm9w5pel", "/2FiY2RlZmdoaWo=", "YWJjZGVmZ2hpauaX", "YWJjZGVmZ2hpau2ggHh5eg=="]
+["abcdefghij�klmnop","abcdefghijéklmnop日","�abcdefghij","abcdefghij�","abcdefghij�xyz"]
+
 @uri
 "\u03bc"
 "%CE%BC"