Clean up trait impls, add some safety comments (#136)

Previously, impls of the `FromBytes`, `AsBytes`, and `Unaligned` traits
were spread across a number of inconsistent and hard-to-follow macro
invocations. This commit cleans that up by introducing a single macro
and a single section of `lib.rs` to hold macro invocations.

This commit also adds some safety comments, but there are still some
left as TODOs. Note that, since this commit does not add or remove any
trait impls (see `test_impls` for a confirmation that the exact set of
impls are tested), these TODOs are acceptable to add in this commit;
this commit moves us strictly forward in terms of safety comment
coverage.

Cargo.toml

@@ -43,5 +43,6 @@ default-features = false
 [dev-dependencies]
 rand = "0.6"
 rustversion = "1.0"
+static_assertions = "1.1"
 # Required for "and $N others" normalization
 trybuild = ">=1.0.70"