Update elm/bytes _Bytes_read_string to use array instead of string as accumulator

[decioferreira]

Relates to #107 and elm/bytes#19.

Updated _Bytes_read_string to use array instead of string as accumulator.

When compiling rtfeldman/elm-css, before changes:

elm-css % rm -rf guida-stuff; time ../guida-lang/compiler/bin/index.js make
Dependencies ready!
Success! Compiled 25 modules.
../guida-lang/compiler/bin/index.js make  21.36s user 2.76s system 101% cpu 23.686 total

After changes:

elm-css % rm -rf guida-stuff; time ../guida-lang/compiler/bin/index.js make
Dependencies ready!
Success! Compiled 25 modules.
../guida-lang/compiler/bin/index.js make  13.95s user 2.20s system 107% cpu 14.984 total

[decioferreira]

Also relates to elm/bytes#25 and elm-janitor/bytes@9a496d7