Add TryFromBytes trait

`TryFromBytes` can be implemented for types which are not `FromZeroes`
or `FromBytes`; it supports performing a runtime check to determine
whether a given byte sequence contains a valid instance of `Self`.

This is the first step of #5. Future commits will add support for a
custom derive and for implementing `TryFromBytes` on unsized types.

TODO:
- More thorough tests for non-FromBytes types (bool, char, etc)
- Tests that go through the `TryFromBytes` public methods rather than
  testing `is_bit_valid` directly
- Update safety requirements for `is_bit_valid` now that it takes a
  `Ptr` rather than a `NonNull`
- Update SAFETY comments in various places

Makes progress on #5

README.md

@@ -19,12 +19,14 @@ so you don't have to.
 
 ## Overview
 
-Zerocopy provides four core marker traits, each of which can be derived
+Zerocopy provides five core marker traits, each of which can be derived
 (e.g., `#[derive(FromZeroes)]`):
 - `FromZeroes` indicates that a sequence of zero bytes represents a valid
   instance of a type
 - `FromBytes` indicates that a type may safely be converted from an
   arbitrary byte sequence
+- `TryFromBytes` supports non-`FromBytes` types by providing the ability
+  to check the validity of a conversion at runtime
 - `AsBytes` indicates that a type may safely be converted *to* a byte
   sequence
 - `Unaligned` indicates that a type's alignment requirement is 1