Check safety of as_raw with a simplified borrow checker (#37)

* wiggle-runtime: add as_raw method for [T]

* add trivial borrow checker back in

* integrate runtime borrow checker with as_raw methods

* handle pointer arith overflow correctly in as_raw, create PtrOverflow error

* runtime: add validation back to GuestType

* generate: impl validate for enums, flags, handles, ints

* oops! make validate its own method on trait GuestTypeTransparent

* fix transparent impls for enum, flag, handle, int

* some structs are transparent. fix tests.

* tests: define byte_slice_strat and friends

* wiggle-tests: i believe my allocator is working now

* some type juggling around memset for ease of use

* make GuestTypeTransparent an unsafe trait

* delete redundant validation of pointer align

* fix doc

* wiggle_test: aha, you cant use sets to track memory areas

* add multi-string test

which exercises the runtime borrow checker against
HostMemory::byte_slice_strat

* oops left debug panic in

* remove redundant (& incorrect, since unchecked) length calc

* redesign validate again, and actually hook to as_raw

* makr all validate impls as inline

this should hopefully allow as_raw's check loop to be unrolled to a
no-op in most cases!

* code review fixes

tests/pointers.rs

@@ -117,10 +117,10 @@ impl PointersAndEnumsExercise {
             )
             .prop_filter("non-overlapping pointers", |e| {
                 MemArea::non_overlapping_set(&[
-                    &e.input2_loc,
-                    &e.input3_loc,
-                    &e.input4_loc,
-                    &e.input4_ptr_loc,
+                    e.input2_loc,
+                    e.input3_loc,
+                    e.input4_loc,
+                    e.input4_ptr_loc,
                 ])
             })
             .boxed()