* lisp/emacs-lisp/map.el: Add support for plists

(map--plist-p, map--plist-delete): New functions.
(map-elt, map-delete, map-length, map-into, map-put!, map-insert)
(map-apply, map-do): Handle the plist case.

* test/lisp/emacs-lisp/map-tests.el (with-maps-do): Add sample plist.
(test-map-put!): The behavior of map-put! is not the same for plists as
for alists.
1 job for master in 2 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #345


Name Stage Failure
test Test There has been a runner system failure, please try again
Running with gitlab-runner 11.4.2 (cf91d5e1)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:4879790bd60d439cfe39c063660eef7af525d5f6f1cbb701a14c7cfc11cbfcf7 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: open /var/run/docker/libcontainerd/containerd/2df34ab366aa1a6e12af214503d89350ba738142c7acb37e6916d43a43501223/init/shim-log.json: no such file or directory (executor_docker.go:1015:0s)