Part of https://github.com/harfbuzz/harfbuzz/issues/1809

Subsetter size goes down from 190kb to 119kb.  Main library about 7kb.

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Part of https://github.com/harfbuzz/harfbuzz/issues/1652

* reimplement ByteStr as byte_str_t based on hb_ubytes_t

Unuse start_embed<ByteStr>
Also renamed SubByteStr to byte_str_ref_t
More renaming to come

* substr renamed to str_ref in line with its type byte_str_ref_t

* uncamelize non-table struct names

* uncamelized non-struct types OpCode etc

* add byte_str_t copy ctor

* test

* test2

* undo tests

* fix bot failure

* undo the previous change

* fixed tabs, added inline

* Revert "fixed tabs, added inline"

This reverts commit 21163c30e9d18759414f7fe2518628241599f039.

* fix tabs

added source hb-ot-cff2-table.cc
augmented VariationData to return scalars
misc bug fixes, renaming, cleanup