This commit adds operations to orc::MemoryAccess for reading basic types
(uint8_t, uint16_t, uint32_t, uint64_t, pointers, buffers, and strings)
from executor memory.
The InProcessMemoryAccess and EPCGenericMemoryAccess implementations are
updated to support the new operations.
This moves the MemoryAccess interface out of the ExecutorProcessControl
class and splits implementation classes InProcessMemoryManager and
SelfExecutorProcessControl out of ExecutorProcessControl.h and into
their own headers.