core: return pointer on gb_read

[deltabeard]

On consecutive reads, it may be faster to obtain a pointer to ROM data on a single call to gb_read, rather than calling gb_read for each byte read.

The same optimisation could also be performed for gb_write.

[deltabeard]

Work being done on https://github.com/deltabeard/Peanut-GB/tree/pointer branch.

[deltabeard]

Reading ROM data is probably inlined by the compiler. Profiling needs to be performed to see if using a pointer instead of a function actually results in any speed improvement.