A few notes:

- this is not all the deletion that we can do, but this is the most
  "simple" case: it leaves in shims, and there's some trivial
  additional cleanup to the transport that can happen but that
  requires writing more code, and I wanted this to be easy to review
  above all else.
  
- This should land *after* we cut the branch for 0.35, but I'm
  anticipating that to happen soon, and I wanted to run this through
  CI.

The layout of struct fields means that interior fields may not be properly
aligned for 64-bit access.

Fixes #7000.

The code in the Tendermint repository makes heavy use of import aliasing.
This is made necessary by our extensive reuse of common base package names, and
by repetition of similar names across different subdirectories.

Unfortunately we have not been very consistent about which packages we alias in
various circumstances, and the aliases we use vary. In the spirit of the advice
in the style guide and https://github.com/golang/go/wiki/CodeReviewComments#imports,
his change makes an effort to clean up and normalize import aliasing.

This change makes no API or behavioral changes. It is a pure cleanup intended
o help make the code more readable to developers (including myself) trying to
understand what is being imported where.

Only unexported names have been modified, and the changes were generated and
applied mechanically with gofmt -r and comby, respecting the lexical and
syntactic rules of Go.  Even so, I did not fix every inconsistency. Where the
changes would be too disruptive, I left it alone.

The principles I followed in this cleanup are:

- Remove aliases that restate the package name.
- Remove aliases where the base package name is unambiguous.
- Move overly-terse abbreviations from the import to the usage site.
- Fix lexical issues (remove underscores, remove capitalization).
- Fix import groupings to more closely match the style guide.
- Group blank (side-effecting) imports and ensure they are commented.
- Add aliases to multiple imports with the same base package name.

This change adds additional coverage to the `mConnConnection.TrySendMessage` code path. Adds test to ensure it returns `io.EOF` when closed.

Addresses: #6570

At Oasis we have spend some time writing a new Ed25519/X25519/sr25519 implementation called curve25519-voi.  This PR switches the import from ed25519consensus/go-schnorrkel, which should lead to performance gains on most systems.

Summary of changes:
 * curve25519-voi is now used for Ed25519 operations, following the existing ZIP-215 semantics.
 * curve25519-voi's public key cache is enabled (hardcoded size of 4096 entries, should be tuned, see the code comment) to accelerate repeated Ed25519 verification with the same public key(s).
 * (BREAKING) curve25519-voi is now used for sr25519 operations.  This is a breaking change as the current sr25519 support does something decidedly non-standard when going from a MiniSecretKey to a SecretKey and or PublicKey (The expansion routine is called twice).  While I believe the new behavior (that expands once and only once) to be more "correct", this changes the semantics as implemented.
 * curve25519-voi is now used for merlin since the included STROBE implementation produces much less garbage on the heap.

Side issues fixed:
 * The version of go-schnorrkel that is currently imported by tendermint has a badly broken batch verification implementation.  Upstream has fixed the issue after I reported it, so the version should be bumped in the interim.

Open design questions/issues:
 * As noted, the public key cache size should be tuned.  It is currently backed by a trivial thread-safe LRU cache, which is not scan-resistant, but replacing it with something better is a matter of implementing an interface.
 * As far as I can tell, the only reason why serial verification on batch failure is necessary is to provide more detailed error messages (that are only used in some unit tests).  If you trust the batch verification to be consistent with serial verification then the fallback can be eliminated entirely (the BatchVerifier provided by the new library supports an option that omits the fallback if this is chosen as the way forward).
 * curve25519-voi's sr25519 support could use more optimization and more eyes on the code.  The algorithm unfortunately is woefully under-specified, and the implementation was done primarily because I got really sad when I actually looked at go-schnorrkel, and we do not use the algorithm at this time.