More efficient constant conversion

[ForNeVeR]

From #157:

Current implementation is inefficient for constants. For example, 1 + 'b' gives the following:

IL_0000: ldc.i4 1
IL_0005: ldc.i4 98
IL_000a: conv.u1
IL_000b: conv.i4
IL_000c: add
IL_000d: stloc V_0

As you see, the char is first converted from int to byte, and then back to int. This could be removed if we implement lowering to implicit casts - the cast expression would see that the operand is a constant and emit it immediately. But this would require a lot of refactoring. so I left it for another day.