Specify bit validity and padding of some types

Specify the bit validity and padding of the primitive numeric 
types, bool, char, and pointer and reference types.

Closes #1291

Author: joshlf

src/type-layout.md

@@ -56,12 +56,23 @@ Most primitives are generally aligned to their size, although this is
 platform-specific behavior. In particular, on x86 u64 and f64 are only
 aligned to 32 bits.
 
+For the primitive numeric types (`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`,
+`i64`, `u128`, `i128`, `usize`, `isize`, `f32`, and `f64`), every bit pattern
+represents a valid instance of the type (in other words,
+`transmute::<[u8; size_of::<T>()], T>(...)` is always sound). For the primitive
+numeric types and also for `bool` and `char`, every byte is guaranteed to be
+initialized (in other words, `transmute::<T, [u8; size_of::<T>()]>(...) is always
+sound).
+
 ## Pointers and References Layout
 
 Pointers and references have the same layout. Mutability of the pointer or
 reference does not change the layout.
 
-Pointers to sized types have the same size and alignment as `usize`.
+Pointers to sized types have the same size and alignment as `usize`. Every
+byte of a pointer to a sized type and of a reference to a sized type is
+initialized (in other words, for such a pointer or reference type, `P`,
+`transmute::<P, [u8; size_of::<P>()]>(...)` is always sound).
 
 Pointers to unsized types are sized. The size and alignment is guaranteed to be
 at least equal to the size and alignment of a pointer.