Add utility functions for string to integer conversions

[chqrlie]

• add is_upper_ascii() and to_upper_ascii()
• add u32toa(), i32toa(), u64toa(), i64toa() u32toa_radix(), u64toa_radix(), i64toa_radix()
• use direct converters instead of snprintf()
• copy NaN and Infinity directly in js_dtoa1()
• optimize js_number_toString() for small ints
• use JS_NewStringLen() instead of JS_NewString() when possible
• add more precise conversion tests in microbench.js

[chqrlie]

Some compilers are not happy...

Yes, gcc 32-bit optimizes the naive copy loop beyond sanity (uses SIMD instructions) which causes reads beyond the array boundaries. I am investigating other approaches. WIP

[chqrlie]

I ran extensive benchmarks to test various methods to convert integers to strings in base 10 or others. You can run make testconv on your system to test with different compilers and options. I changed the variant used in cutils.c for better performance and to avoid some nasty over-optimisations by gcc on some CI targets. The variant selected shifts 1 to 7 digits into a 64-bit register and writes them as an 8-byte block including the null terminator. This is quite fast, albeit not as fast as composing the string 2 digits at a time, which uses more memory and causes gcc to output warnings for what seems to be an optimisation issue.