Fix #597; use SAVE-FILE to visit compiled snippets
Rename the existing FILE field to LOAD-FILE, and add a new SAVE-FILE to
yas--template struct. Normally they will be the same, but compiled
snippets have only SAVE-FILE because they are loaded by "compiled" code.
* yasnippet.el (yas--template): Remove field file, add load-file and
save-file.
(yas--parse-template): Update snippet-definition list in docstrings.
(yas-define-snippets): Move LOAD-FILE value to SAVE-FILE when compiling.
(yas--template-get-file): New function.
(yas--visit-snippet-file-1, yas-load-snippet-buffer,
yas-load-snippet-buffer-and-close): Use yas--template-load-file,
yas--template-save-file, or yas--template-get-file, as appropriate.
* doc/snippet-organization.org (The =.yas-compiled-snippet.el= file):
Remove caveat that is no longer relevant.