libutils: Improve performance of utf8_to_utf16/utf16_to_utf8

This CL improves the performance of below functions in helping with conversion
between utf8/utf16 with libutils:

  - utf8_to_utf16_length
  - utf8_to_utf16
  - utf16_to_utf8_length
  - utf16_to_utf

The basic idea is to keep the loop as tight as possible for the most
common cases, e.g. in UTF16-->UTF8 case, the most common case is
when the character is < 0x80 (ASCII), next is when it's < 0x0800 (
most Latin), and so on.

This version of implementation reduces the number of instructions
needed for every incoming utf-8 bytes in the original implementation
where:

  1) calculating how many bytes needed given a leading UTF-8 byte
     in utf8_codepoint_len(), it's a very clever way but involves
     multiple instructions to calculate regardless

  2) and an intermediate conversion to utf32, and then to utf16
     utf8_to_utf32_codepoint()

The end result is about ~1.5x throughput improvement.

Benchmark results on redfin (64bit) before the change:

utf8_to_utf16_length: bytes_per_second=307.556M/s
utf8_to_utf16:        bytes_per_second=246.664M/s
utf16_to_utf8_length: bytes_per_second=482.241M/s
utf16_to_utf8:        bytes_per_second=351.376M/s

After the change:

utf8_to_utf16_length: bytes_per_second=544.022M/s
utf8_to_utf16:        bytes_per_second=471.135M/s
utf16_to_utf8_length: bytes_per_second=685.381M/s
utf16_to_utf8:        bytes_per_second=580.004M/s

Ideas for future improvement could include alignment handling and loop
unrolling to increase throughput more.

This CL also fixes issues below:

  1. utf16_to_utf8_length() should return 0 when the source string has
     length of 0, the original code returns -1 as below:

    ssize_t utf16_to_utf8_length(const char16_t *src, size_t src_len)
    {
        if (src == nullptr || src_len == 0) {
            return -1;
        }
	...

  2. utf8_to_utf16() should check whether input string is valid.

Change-Id: I546138a7a8050681a524eabce9864219fc44f48e