Make sure an indirect jump never uses an address beginning on the last byte of a page

[munshkr]

There is a bug in the 6502 where an indirect JMP will never cross page boundaries. E.g: If address $3000 contains $40, $30FF contains $80, and $3100 contains $50, the result of JMP ($30FF) will be a transfer of control to $4080 rather than $5080.

Assembler should do both of these:

• If JMP uses a label that references an address on the last byte of a page (i.e. low byte of address is $ff), shift up or down code referenced by label 1 byte.
• If JMP uses a hardcoded address that is on the last byte of a page, throw a warning.