The old hb-ot-tag.cc functions, `hb_ot_tags_from_script` and
`hb_ot_tag_from_language`, are now wrappers around a new function:
`hb_ot_tags`. It converts a script and a language to arrays of script
tags and language tags. This will make it easier to add new script tags
to scripts, like 'dev3'. It also allows for language fallback chains;
nothing produces more than one language yet though.

Where the old functions return the default tags 'DFLT' and 'dflt',
`hb_ot_tags` returns an empty array. The caller is responsible for
using the default tag in that case.

The new function also adds a new private use subtag syntax for script
overrides: "x-hbscabcd" requests a script tag of 'abcd'.

The old hb-ot-layout.cc functions,`hb_ot_layout_table_choose_script` and
`hb_ot_layout_script_find_language` are now wrappers around the new
functions `hb_ot_layout_table_select_script` and
`hb_ot_layout_script_select_language`. They are essentially the same as
the old ones plus a tag count parameter.

Closes #495.