* Add utility functions, improve integer conversion functions
- move `is_be()` to cutils.h
- add `is_upper_ascii()` and `to_upper_ascii()`
- add extensive benchmark for integer conversion variants in **tests/test_conv.c**
- add `u32toa()`, `i32toa()`, `u64toa()`, `i64toa()` based on register shift variant
- add `u32toa_radix()`, `u64toa_radix()`, `i64toa_radix()` based on length_loop variant
- use direct converters instead of `snprintf()`
- copy NaN and Infinity directly in `js_dtoa1()`
- optimize `js_number_toString()` for small integers
- use `JS_NewStringLen()` instead of `JS_NewString()` when possible
- add more precise conversion tests in microbench.js
- disable some benchmark tests for gcc (they cause ASAN failures)
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