Bug fixes and builtins

The central theme of this release is bug fixes, most notably in the cue command line utility.
It also adds and improves on some of the builtins in the pkg directory.

Backwards incompatible changes

The crypto hash builtins now return a value of type bytes, instead of a list of bytes.

Changelog

cb84024 cmd/cue/cmd: fix --dryrun flag description of import command
fe695b1 cmd/cue: give access to cue Command in run functions
9818761 cmd/cue: update get to use definitions
507d6cc cmd/cue: use concrete mode for vet on data files
f3df637 cmd/cue: use stderr and non-zero exit for errors
330999b cue/pkg/crypto: make return types bytes, instead of list
1c4d597 cue/scanner: don't allow repeated _ in numbers
18637db cue: closedness only applies to regular fields
5e8c391 cue: exempt aliases and embeddings from recursive closing
34988fa cue: fix 0/0
4017875 doc/ref: spec changes to comprehensions and identifiers
da39f0f doc/tutorial/basic: Unified TOC, next/prev, titles
980e018 doc/tutorials/kubernetes: update comments
8b4cab6 doc: fixed small errors
d2563c1 go.mod: update xerrors to a985d3407aa7
41e30c6 pkg/list: adding aggregation for decimal lists
9d5bc94 pkg/strings: Remove fmt.Println
b36c351 pkg/strings: fix MinRunes
061bde1 spec: corrections, clarifications, comments