Adds InstructionDecoder class

It is a helper class that performs instruction decoding
in an explicit manner. Previous implementation were
encapsulated in TSmalltalkInstruction::decodeBytecode()
and corresponding constructor which is not intuitive.

Explicit use of InstructionDecoder is recomended in places
that iterate through bytecode instructions one by one.

Issue: #32
Refactors software VM to use InstructionDecoder

Issue: #32
Refactors ParsedBytecode to use InstructionDecoder

Issue: #32
Removes obsolete TSmalltalkInstruction::decodeBytecode() and constructor

Issue: #32