Implement SETDP pseudo operation

[craigthomas]

Describe the enhancement
The SETDP pseudo-operation is used to tell the assembler to optimize code. Specifically, the SETDP mnemonic allows the programmer to specify what the contents of the direct page register are during compilation. The assembler can then look for any addresses during assembly that have a most significant byte the same as what is stored in the direct page register. It will then transform those statements from their extended addressing mode equivalents into direct addressing mode statements instead.

For example, normally:

LDA $0F01

Would generate the following machine code:

B6 0F 01

However, if the direct page were loaded with $0F, and the SETDP pseudo-operation were implemented, then:

SETDP $0F
LDA $0F01

Would generate the following machine code:

96 01