uefi: mem.rs -> mem/{mod, util}.rs

This is a preliminary measurement to move the types for the uefi memory map (et al.)
from the boot services module to this module.

Author: phip1611

uefi/src/mem/mod.rs

@@ -0,0 +1,5 @@
+//! Types, functions, traits, and other helpers to work with memory in UEFI
+//! libraries and applications.
+
+#[cfg(feature = "alloc")]
+pub(crate) mod util;