Define special key for asynchronous events

K_EVENT/KE_EVENT are used to signal any loop that reads user input(scattered
across normal.c edit.c , ex_getln.c and message.c) of asynchronous events that
were not initiated by the user.

Representing non-user asynchronous events as special keys has the following
advantages:

- We reuse the normal vim redrawing code. As far as the rest of the code in
  edit.c/normal.c is concerned, it's just the user pressing another key.
- Assume less about vim tolerance for "out-of-band" modifications to its
  internal state.
- We still have a very complex codebase and it's hard to predict what bugs may
  be introduced by these changes. With this we implement asynchronicity in a way
  that will be more "natural" to the editor and has less chance of causing
  unpredictable behavior.

As the code is refactored, we will be able to treat user input as an 'event
type' and not the other way around(With this we are treating arbitrary events as
a special case of user input).
Remove old comment from job.c
Remove donation value from backers.md
Sort backers.md
Add some missing backers and remove duplicates
Move functions from os/fs.c into path.c

Move unit tests from os/fs.moon to path.moon
Move and adapt `os_get_absolute_path` unit tests to `vim_FullName`

* Add two new unit tests to `vim_FullName`
* Make `os_get_absolute_path` static
Change prefix from `os_*` to `path_*`
Move exe functions back to os/fs.c
vim-patch:7.4.186

Problem:    Insert in Visual mode sometimes gives incorrect results.
            (Dominique Pelle)
Solution:   Remember the original insert start position. (Christian Brabandt,
            Dominique Pelle)

https://code.google.com/p/vim/source/detail?r=4d12112c5efae071aecbeed1a7196f18950457b3
vim-patch:7.4.187

Problem:    Delete that crosses line break splits multi-byte character.
Solution:   Advance a character instead of a byte. (Cade Foster)

https://code.google.com/p/vim/source/detail?r=a1c07956171a133583df42627d3498f935e59988