[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 */

src/sim65/main.c

@@ -177,10 +177,16 @@ static unsigned char ReadProgramFile (void)
 
     /* Get the CPU type from the file header */
     if ((Val = fgetc(F)) != EOF) {
-        if (Val != CPU_6502 && Val != CPU_65C02) {
+        switch (Val) {
+        case CPU_6502:
+        case CPU_65C02:
+        case CPU_6502X:
+            CPU = Val;
+            break;
+
+        default:
             Error ("'%s': Invalid CPU type", ProgramFile);
         }
-        CPU = Val;
     }
 
     /* Get the address of sp from the file header */