Adds an opcode definition, instruction representation and decoder for TrueType bytecode.

Also renames Args -> InlineOperands and moves the type from skrifa to read-fonts.

Based on #773, uses the decoder in that PR to handle dispatch to the currently implemented instructions.

No tests in this one. Will add in a later PR when more functionality has landed.

- add limit for total number of instructions executed (same as FT)
- don't return count from Engine::run()

- move bytecode to glyf module
- replace negation with abs()
- make DecodeError a unit-like struct

In addition, replace byte to Opcode table with a match expression. This codegens the same as transmute!