reg: support for register casting

Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().

Closes #1

script/generate

@@ -1,8 +1,8 @@
 #!/bin/bash -ex
 
 # Separate core packages from those that depend on the whole library being built.
-core=$(go list ./... | grep -v examples)
-post=$(go list ./... | grep examples)
+core=$(go list ./... | grep -Ev 'avo/(examples|tests)')
+post=$(go list ./... | grep -E  'avo/(examples|tests)')
 
 # Install avogen (for bootstrapping).
 go install ./internal/cmd/avogen