tka: implement AUM and Key types

This is the first in a series of PRs implementing the internals for the
Tailnet Key Authority. This PR implements the AUM and Key types, which
are used by pretty much everything else. Future PRs:

 - The State type & related machinery
 - The Tailchonk (storage) type & implementation
 - The Authority type and sync implementation

Signed-off-by: Tom DNetto <tom@tailscale.com>

go.mod

@@ -69,6 +69,7 @@ require (
 
 require (
 	4d63.com/gochecknoglobals v0.1.0 // indirect
+	filippo.io/edwards25519 v1.0.0-rc.1 // indirect
 	github.com/Antonboom/errname v0.1.5 // indirect
 	github.com/Antonboom/nilnil v0.1.0 // indirect
 	github.com/BurntSushi/toml v1.1.0 // indirect
@@ -121,6 +122,7 @@ require (
 	github.com/fatih/color v1.13.0 // indirect
 	github.com/fatih/structtag v1.2.0 // indirect
 	github.com/fsnotify/fsnotify v1.5.1 // indirect
+	github.com/fxamacker/cbor/v2 v2.4.0 // indirect
 	github.com/fzipp/gocyclo v0.3.1 // indirect
 	github.com/gliderlabs/ssh v0.3.3 // indirect
 	github.com/go-critic/go-critic v0.6.1 // indirect
@@ -162,6 +164,7 @@ require (
 	github.com/hashicorp/errwrap v1.1.0 // indirect
 	github.com/hashicorp/go-multierror v1.1.1 // indirect
 	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/hdevalence/ed25519consensus v0.0.0-20220222234857-c00d1f31bab3 // indirect
 	github.com/huandu/xstrings v1.3.2 // indirect
 	github.com/imdario/mergo v0.3.12 // indirect
 	github.com/inconshreveable/mousetrap v1.0.0 // indirect
@@ -255,6 +258,7 @@ require (
 	github.com/uudashr/gocognit v1.0.5 // indirect
 	github.com/vbatts/tar-split v0.11.2 // indirect
 	github.com/vishvananda/netns v0.0.0-20211101163701-50045581ed74 // indirect
+	github.com/x448/float16 v0.8.4 // indirect
 	github.com/xanzy/ssh-agent v0.3.1 // indirect
 	github.com/yeya24/promlinter v0.1.0 // indirect
 	go4.org/intern v0.0.0-20211027215823-ae77deb06f29 // indirect