Add support for instruction "aliases"

[S3v3ru5]

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}