performance improvements: cache errors and warnings explicitly

There are many calls to s:Errors() and s:Warnings(), and previously the
location list was filtered once for each of these calls. This made vim
unusable when handling large location lists (with hundreds of entries).
Now we cache the errors and warnings explicitly when we cache the
location list.

Rework some of the minor methods so that they call s:Errors() or
s:Warnings() (which use the cached data) instead of calling
s:FilterLocList() directly (which is expensive).

plugin/syntastic.vim

@@ -160,8 +160,10 @@ function! s:LocList()
 endfunction
 
 "clear the loc list for the buffer
-function! s:ClearLocList()
+function! s:ClearCache()
     let b:syntastic_loclist = []
+    unlet! b:syntastic_errors
+    unlet! b:syntastic_warnings
 endfunction
 
 "detect and cache all syntax errors in this buffer
@@ -169,7 +171,7 @@ endfunction
 "depends on a function called SyntaxCheckers_{&ft}_GetLocList() existing
 "elsewhere
 function! s:CacheErrors()
-    call s:ClearLocList()
+    call s:ClearCache()
 
     if filereadable(expand("%"))
 
@@ -195,7 +197,7 @@ function! s:ToggleMode()
         let g:syntastic_mode_map['mode'] = "active"
     endif
 
-    call s:ClearLocList()
+    call s:ClearCache()
     call s:UpdateErrors(1)
 
     echo "Syntastic: " . g:syntastic_mode_map['mode'] . " mode enabled"
@@ -217,30 +219,22 @@ function! s:ModeMapAllowsAutoChecking()
     endif
 endfunction
 
-"return true if there are cached errors/warnings for this buf
-function! s:BufHasErrorsOrWarnings()
-    return !empty(s:LocList())
-endfunction
-
-"return true if there are cached errors for this buf
-function! s:BufHasErrors()
-    return len(s:ErrorsForType('E')) > 0
-endfunction
-
 function! s:BufHasErrorsOrWarningsToDisplay()
-    return s:BufHasErrors() || (!g:syntastic_quiet_warnings && s:BufHasErrorsOrWarnings())
+    return len(s:Errors()) || (!g:syntastic_quiet_warnings && !empty(s:LocList()))
-endfunction
-
-function! s:ErrorsForType(type)
-    return s:FilterLocList({'type': a:type})
 endfunction
 
 function! s:Errors()
-    return s:ErrorsForType("E")
+    if !exists("b:syntastic_errors")
+        let b:syntastic_errors = s:FilterLocList({'type': "E"})
+    endif
+    return b:syntastic_errors
 endfunction
 
 function! s:Warnings()
-    return s:ErrorsForType("W")
+    if !exists("b:syntastic_warnings")
+        let b:syntastic_warnings = s:FilterLocList({'type': "W"})
+    endif
+    return b:syntastic_warnings
 endfunction
 
 "Filter a loc list (defaults to s:LocList()) by a:filters