core: Flatten public module structure

This patch makes some modules of the core crate private and re-exports
their contents from the crate root.  This has two benefits:  It makes
it easier to read the crate documentation because the most important
types are directly visible from the root.  And it makes the imports for
the most important types less verbose.

See also: #155

core/src/client.rs

@@ -7,34 +7,19 @@ use crate::{
 };
 
 #[cfg(feature = "attestation-client")]
-mod attestation;
+pub mod attestation;
 #[cfg(feature = "certificate-client")]
-mod certificate;
+pub mod certificate;
 #[cfg(feature = "counter-client")]
-mod counter;
+pub mod counter;
 #[cfg(feature = "crypto-client")]
-mod crypto;
+pub mod crypto;
 #[cfg(feature = "filesystem-client")]
-mod filesystem;
+pub mod filesystem;
 #[cfg(feature = "management-client")]
-mod management;
+pub mod management;
 #[cfg(feature = "ui-client")]
-mod ui;
-
-#[cfg(feature = "attestation-client")]
-pub use attestation::AttestationClient;
-#[cfg(feature = "certificate-client")]
-pub use certificate::CertificateClient;
-#[cfg(feature = "counter-client")]
-pub use counter::CounterClient;
-#[cfg(feature = "crypto-client")]
-pub use crypto::*;
-#[cfg(feature = "filesystem-client")]
-pub use filesystem::FilesystemClient;
-#[cfg(feature = "management-client")]
-pub use management::ManagementClient;
-#[cfg(feature = "ui-client")]
-pub use ui::UiClient;
+pub mod ui;
 
 // to be fair, this is a programmer error,
 // and could also just panic