Restrict `HTMLParser` interface to only accept binaries

[philss]

This makes the interfaces simplier. We don't need to support iodata for now yet.

[philss]

cc/ @wojtekmach

[ypconstante]

iodata support would be useful on Phoenix LiveView tests, for example. Today Phoenix has the content stored as iodata and converts to binary just to call parse, and because mochi_html adds the <floki> root element, for each parse we ended up with two identical binaries being created.