Refactor stack IR / binary writer (NFC) (#2250)

Previously `StackWriter` and its subclasses had routines for all three
modes (`Binaryen2Binary`, `Binaryen2Stack`, and `Stack2Binary`) within a
single class. This splits routines for each in a separate class and
also factors out binary writing into a separate class
(`BinaryInstWriter`) so other classes can make use of it.

The new classes are:
- `BinaryInstWriter`:
   Binary instruction writer. Only responsible for emitting binary
   contents and no other logic
- `BinaryenIRWriter`: Converts binaryen IR into something else
  - `BinaryenIRToBinaryWriter`: Writes binaryen IR to binary
  - `StackIRGenerator`: Converts binaryen IR to stack IR
- `StackIRToBinaryWriter`: Writes stack IR to binary

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