memory: inline some performance-sensitive accessors

memory: inline some performance-sensitive accessors

These accessors are called from inlined functions, and the call sequence
is much more expensive than just inlining the access.  Move the
struct declaration to memory-internal.h so that exec.c and memory.c
can both use an inline function.

Reviewed-by: Alexey Kardashevskiy <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>