Flatten API (#274)

Re-exports types from the toplevel instead of placing them in individual
modules (often which only contain one type).

This makes the API easier for users to navigate, while still retaining
the same module structure internally.

Additionally, this commit uses the `uuid` crate for modeling UUIDs.

src/policy.rs

@@ -3,8 +3,9 @@
 use crate::{serialization::Tlv, Result};
 
 /// Specifies how often the PIN needs to be entered for access to the credential in a
-/// given slot. This policy must be set upon key generation or importation, and cannot be
-/// changed later.
+/// given slot.
+///
+/// This policy must be set when keys are generated or imported, and cannot be changed later.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum PinPolicy {
     /// Use the default PIN policy for the slot. See the slot's documentation for details.
@@ -46,8 +47,9 @@ impl PinPolicy {
 }
 
 /// Specifies under what conditions a physical touch on the metal contact is required, in
-/// addition to the [`PinPolicy`]. This policy must be set upon key generation or
-/// importation, and cannot be changed later.
+/// addition to the [`PinPolicy`].
+///
+/// This policy must be set when keys are generated or imported, and cannot be changed later.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum TouchPolicy {
     /// Use the default touch policy for the slot.