Various bug fixes

This release includes various bug fixes and error message improvements.

Changelog

d5041a1 cmd/cue/cmd: better checks for incorrect flags
858efa9 cue/ast/astutil: fix resolution bugs
5b8ab47 cue: elide comma after ...
3fdad16 cue: move field name to end of error message
dd188a6 cue: move more labels and values to end of error message
48ddb2b doc/ref/spec.md: fix typos
30abd66 internal/core/adt: fix typo
cd286a8 internal/core/export: fix export of disjunction with eliminated defaults
7180c7d pkg/tool/http: fill in status and statusCode
073c6aa pkg/tool/http: make request.body optional