Add cache flush in py/asmarm.c and add new MP_PLAT_ALLOC_EXEC and MP_PLAT_FREE_EXEC macros

Fixes issue #840
Basic nspire support
nsp module

-Texture class
-waitKeypress function
-sys.exit exits cleanly
-3MiB heap
Add nsp.readRTC() function and turn on LTO and -Ofast
Get rid of unnecessary __del__
Add greyscale support (needs modified nspire-io)