[SIM65] Support undocumented opcodes for 6502

This PR is mostly a complete rewrite of the emulator for 6502/65c02 opcodes.
It provides an easier to maintain implementation of the instructions, by using few macros rather than having hand-written code for each function.
All undocumented, previously missing opcodes for 6502 are also implemented.
The patch also includes a detailed documentation of those opcodes, for reference to developers.
This PR should fix one of the milestones listed here for the next version of CC65:

https://github.com/cc65/wiki/wiki/Before-the-next-release

src/sim65/6502.h

@@ -47,7 +47,8 @@
 /* Supported CPUs */
 typedef enum CPUType {
     CPU_6502,
-    CPU_65C02
+    CPU_65C02,
+    CPU_6502X
 } CPUType;
 
 /* Current CPU */