[wasm] Option to implement OCaml strings with JavaScript strings

[vouillon]

• Conditional compilation of Wasm runtime
• Import string constants (define JS strings in Wasm runtime)
• default should be not to use js strings
• Compatibilty for when JS string builtins proposal is not available
• Performance improvements
  • skip conversions for ASCII strings
  • lower level version of caml_blit_string
  • Use I16 array for conversions
  • Remove unnecessary conversion
• clean-up / check for missing tests
• adapt Jane street libraries
• performance measurements