Move executable existence check to platform utils
Move complete_cmd_in_path() to utils/path
Change {complete,find}_cmd_in_path()

This allows for checking for an executable without obtaining path
to it.
Add executable() builtin function

Checks whether executable exists in directories listed in $PATH.  Checks for
various executable extensions on Windows.
Document executable() builtin function

Checks whether executable exists in one of directories listed in
$PATH.
Merge branch 'add-executable-builtin-function'

Add executable() builtin function that checks whether executable
exists in one of directories listed in $PATH.