Implement a single local variable

[jonathanpeppers]

Based on attributetable, but with the change:

++// For testing locals
++uint fill = 32*30;
++
// set background palette colors
pal_bg(PALETTE);

// fill nametable with diamonds
vram_adr(NAMETABLE_A);  // start address ($2000)
--vram_fill(0x16, 32 * 30);   // fill nametable (960 bytes)
++vram_fill(0x16, fill);   // fill nametable (960 bytes)