Wasm backend

towards #583

• As suggested by @certik , it uses parallel streams and functions declared in WASMAssembler namespace.
• the member variable names of ASRToWASMVisitor currently (temporarily) uses m_ prefix. (It can be removed, if suggested/advised)
• In the hope of improving performance, the functions encode_signed_leb128 and encode_unsigned_leb128 have been defined twice
  • once as encode_signed_leb128 and encode_unsigned_leb128
  • and next time as emit_signed_leb128 and emit_unsigned_leb128