MMIO: Lazy-initialize read/write handlers since most are not actually used

Decreases time taken to create an MMIO Mapping object dramatically.

Performance before:

    [----------] Global test environment tear-down
    [==========] 5 tests from 3 test cases ran. (472 ms total)
    [  PASSED  ] 5 tests.

Performance after:

    [----------] Global test environment tear-down
    [==========] 5 tests from 3 test cases ran. (43 ms total)
    [  PASSED  ] 5 tests.