Improve internal string allocation methods (#398)

String values are allocated as temporary or final results. This commit
attempts to improve the consistency and performance of this step.

- define `JS_NewString` as an inline function to allow simple expansion
  of `strlen()` for string literals
- document string contents constraints regarding UTF-8 encoding.
- rename `js_new_string8` as `js_new_string8_len`. takes `const char *`.
- new inline function `js_new_string8` takes `const char *`, computes
  string length with `strlen` and calls `js_new_string8_len`. No overhead
  for string literals
- rename `js_new_string16` to `js_new_string16_len`
- use internal string allocation functions where appropriate, remove overhead
- allocate extra byte for null terminator in source code string

Author: chqrlie