instead read them from the `libc` crate if available.

fixes #216

Memory::read_ptr has to check for relocations on the edges

Produce `ConstInt` from a `def_id` for rustc

Add tests for #113

resolves #113

Add little script to build libstd

hooking mmap is no longer needed

That's easier to use than having to `cd xargo`

Use a wrapper type to differentiate between `PrimVal` and pointers

Update for allcator API, simplify reallocate

Make Reallocate & Deallocate less permissive

Only check pointers when dereferencing

Implement some missing intrinsics

Fix transmute on ByValPair

Fixes #227