Implements the ldelema opcode
Removes hardcoded test scenarios
Implements string.Concat() method

string.Concat(string, string, string) overload
Implements ldind.ref and stind.ref opcodes
Fixes a clause
Partially supports abstract method calls

Can still suffer from bad order of arguments.
Supports abstract method calls

Replaces the previous commit with a new logic that should handle all
abstract method calls all right. Also changed the way how method
arguments are passed to a call so that it better correspond to a way
described on MSDN.