* Implement thread-safe cloning of a secret connection

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Expand documentation for SecretConnection on threading considerations

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Extract peer construction into its own method

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Add test for cloned SecretConnection

This adds a `TcpStream`-based test for parallelizing operations on a
`SecretConnection`. I used `TcpStream` instead of the buffered reader in
the other tests because it wasn't feasible to implement the `TryClone`
trait for that buffered pipe implementation.

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Add more messages to test

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Expand comment for clarity

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Add .changelog entry

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Restore half-duplex operations

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Extract encrypt/decrypt fns as independent methods

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Remove unnecessary trait bounds

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Extract send/receive state

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Extract read/write functionality as standalone methods

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Add logic to facilitate splitting SecretConnection into its sending and receiving halves

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Restore split SecretConnection test using new semantics

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Update changelog entry

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Update docs for `SecretConnection`

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Condense error reporting

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Extract TryClone trait into its own crate

As per the discussion at
#938 (comment),
this extracts the `TryClone` trait into a new crate called
`tendermint-std-ext` in the `std-ext` directory.

This new crate is intended to contain any code that we need that extends
the Rust standard library.

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Reorder imports

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Assert validation regardless of debug build

This introduces the internal encryption assertions at runtime regardless
of build type. This may introduce a small performance hit, but it's
probably worth it to ensure correctness.

Effectively this is keeping an eye on the code in the
`encrypt_and_write` fn to ensure its correctness.

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Remove remote_pubkey optionality from sender/receiver halves

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Update SecretConnection docs with comment content

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Fix doc link to TryClone trait

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Fix doc link to TryClone trait

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Add docs on SecretConnection failures and connection integrity

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Synchronize sending/receiving failures to comply with crypto algorithm constraints

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Rename try_split method to split for SecretConnection

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Remove redundant field name prefixes

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Fix broken link in docs

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Fix needless borrows in codebase

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Ignore needless collect warning (we do actually seem to need it)

Signed-off-by: Thane Thomson <connect@thanethomson.com>

* Remove trailing semicolon in macro to fix docs compiling

Signed-off-by: Thane Thomson <connect@thanethomson.com>

Signed-off-by: Thane Thomson <connect@thanethomson.com>