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Fixes #264.
Code review: Simpler prop syntax and removed a function
Merge pull request #480 from rojepp/code_templates

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Only parse the current buffer if it was modified

A new buffer-local variable (fsharp-ac-last-parsed-ticks) is used the
keep track of BUFFER's tick counter at the moment when the buffer was
last parsed.
Make fsharp-ac-errors buffer-local in fsharp-mode buffers

The previous use of make-local-variable had no effect, because it made
the variable buffer-local only for current buffer at the time when
"fsharp-mode-completion.el" was loaded and not for future fsharp-mode
buffers.