v0.4.0-rc.1

This release includes various bug fixes.

Changelog

393ec28 cue/cmd/cue: fix regression for -H flag
13a4d3c encoding/openapi: finalise when constructing openapi AST
287d43c internal/core/export: don't wrap embedded scalars in _#def
1e14710 internal/third_party/yaml: fix faulty decoding of hidden fields