Add support for instruction "aliases"

Teal assembler supports various alias instructions which are not mentioned in the Opcodes document.

- `method {s: method-signature}`: Alias to `bytes {4-byte method selector}`
- `extract` instruction without immediate arguments is equivalent to `extract3`.
- `txn` instruction with 2 immediate arguments(`txn {i} {j}` is equivalent to `txna {i} {j}`
- `gtxn` instruction with 3 immediate arguments(`gtxn {i} {j} {k}`) is equivalent to `gtxna {i} {j} {k}`
- `gtxns` instruction with 2 immediate arguments(`gtxns {i} {j}`) is equivalent to `gtxnsa {i} {j}`