admit we don't have a memory model

src/memory-model.md

@@ -1,10 +1,5 @@
 # Memory model
 
-A Rust program's memory consists of a static set of *items* and a *heap*.
-Immutable portions of the heap may be safely shared between threads, mutable
-portions may not be safely shared, but several mechanisms for effectively-safe
-sharing of mutable values, built on unsafe code but enforcing a safe locking
-discipline, exist in the standard library.
-
-Allocations in the stack consist of *variables*, and allocations in the heap
-consist of *boxes*.
+Rust does not yet have a defined memory model. Various academics and industry
+are working on various proposals, but for now, this is an under-defined place
+in the language.