[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

libsrc/sim6502/exehdr.s

@@ -9,11 +9,21 @@
         .import         __MAIN_START__
         .import         startup
 
+        .macpack        cpu
+
         .segment        "EXEHDR"
 
         .byte   $73, $69, $6D, $36, $35        ; 'sim65'
         .byte   2                              ; header version
-        .byte   .defined(__SIM65C02__)         ; CPU type
+.if (.cpu .bitand ::CPU_ISET_6502X)
+        .byte   2
+.elseif (.cpu .bitand ::CPU_ISET_65C02)
+        .byte   1
+.elseif (.cpu .bitand ::CPU_ISET_6502)
+        .byte   0
+.else
+        .error Unknow CPU type.
+.endif
         .byte   sp                             ; sp address
         .addr   __MAIN_START__                 ; load address
         .addr   startup                        ; reset address