etc: Add snippets.

* etc/snippets/scheme-mode/guix-cvs-reference,
etc/snippets/scheme-mode/guix-git-reference,
etc/snippets/scheme-mode/guix-hg-reference,
etc/snippets/scheme-mode/guix-origin,
etc/snippets/scheme-mode/guix-package,
etc/snippets/scheme-mode/guix-svn-reference,
etc/snippets/text-mode/guix-commit-message-add-package,
etc/snippets/text-mode/guix-commit-message-update-package: New files.
* doc/contributing.texi (The Perfect Setup): Document snippets.

etc/snippets/scheme-mode/guix-cvs-reference

@@ -0,0 +1,8 @@
+# -*- mode: snippet -*-
+# name: guix-cvs-reference
+# key: cvs-reference...
+# --
+(cvs-reference
+  (root-directory "${1:root-directory}")
+  (module "${2:module}")
+  (revision "${3:revision}"))

etc/snippets/scheme-mode/guix-git-reference

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: guix-git-reference
+# key: git-reference...
+# --
+(git-reference
+  (url "$1")
+  (commit ${2:commit}))

etc/snippets/scheme-mode/guix-hg-reference

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: guix-hg-reference
+# key: hg-reference...
+# --
+(hg-reference
+  (url "$1")
+  (changeset ${2:changeset}))

etc/snippets/scheme-mode/guix-origin

@@ -0,0 +1,23 @@
+# -*- mode: snippet -*-
+# name: guix-origin
+# key: origin...
+# --
+(origin
+ (method ${1:$$(yas-choose-value "url-fetch"
+                                 "url-fetch/tarbomb"
+                                 "url-fetch/zipbomb"
+                                 "cvs-fetch"
+                                 "git-fetch"
+                                 "hg-fetch"
+                                 "svn-fetch")})
+ (uri ${1:$(cond ((equal yas-text "git-fetch") "git-reference...")
+                 ((equal yas-text "svn-fetch") "svn-reference...")
+                 ((equal yas-text "hg-fetch")  "hg-reference...")
+                 ((equal yas-text "cvs-fetch") "cvs-reference...")
+                 (t "(string-append \\"https://\\" version \\".tar.gz\\")"))}$0)
+ ${1:$(cond ((member yas-text '("git-fetch" "svn-fetch" "hg-fetch" "cvs-fetch"))
+             "(file-name (string-append name \\"-\\" version \\"-checkout\\"))")
+            (t ""))}
+ (sha256
+  (base32
+    "$2")))

etc/snippets/scheme-mode/guix-package

@@ -0,0 +1,34 @@
+# -*- mode: snippet -*-
+# name: guix-package
+# key: package...
+# --
+(define-public $1
+  (package
+    (name "$1")
+    (version "$2")
+    (source origin...$0)
+    (build-system ${3:$$(yas-choose-value "ant-build-system"
+                                          "asdf-build-system"
+                                          "cargo-build-system"
+                                          "cmake-build-system"
+                                          "dub-build-system"
+                                          "emacs-build-system"
+                                          "font-build-system"
+                                          "glib-or-gtk-build-system"
+                                          "gnu-build-system"
+                                          "go-build-system"
+                                          "haskell-build-system"
+                                          "meson-build-system"
+                                          "minify-build-system"
+                                          "ocaml-build-system"
+                                          "perl-build-system"
+                                          "python-build-system"
+                                          "r-build-system"
+                                          "ruby-build-system"
+                                          "texlive-build-system"
+                                          "trivial-build-system"
+                                          "waf-build-system")})
+    (home-page "$4")
+    (synopsis "$5")
+    (description "$6")
+    (license $7)))

etc/snippets/scheme-mode/guix-svn-reference

@@ -0,0 +1,7 @@
+# -*- mode: snippet -*-
+# name: guix-svn-reference
+# key: svn-reference...
+# --
+(svn-reference
+  (url "$1")
+  (revision ${2:svn-revision}))