use_version() increments the "Version" field in DESCRIPTION,
adds a new heading to NEWS.md (if it exists), and commits those changes
(if package uses Git). It makes the same update to a line like PKG_version = "x.y.z"; in src/version.c (if it exists).
use_dev_version() increments to a development version, e.g.
from 1.0.0 to 1.0.0.9000. If the existing version is already a development
version with four components, it does nothing. Thin wrapper around
use_version().