Add support for context aware completion

If complete operation is given context property containing the
current toplevel form Compliment library can give better completion
results in some contexts:

- Local vars for let and defn
- Resources in classpath for io/resource calls
- Vars in specific ns for :require :refer
- Better Java class name completion for :import

The context string should contain symbol __prefix__ in place of the word
being completed. To achieve this location of cursor inside the toplevel
form is calculated so that the placeholder symbol can be placed in the
proper place.

plugin/fireplace.vim

@@ -108,6 +108,37 @@ function! s:candidate(val) abort
         \ }
 endfunction
 
+function! s:get_complete_context() abort
+  " Find toplevel form
+  " If cursor is on start parenthesis we don't want to find the form
+  " If cursor is on end parenthesis we want to find the form
+  let [line1, col1] = searchpairpos('(', '', ')', 'Wrnb', g:fireplace#skip)
+  let [line2, col2] = searchpairpos('(', '', ')', 'Wrnc', g:fireplace#skip)
+
+  if (line1 == 0 && col1 == 0) || (line2 == 0 && col2 == 0)
+    return ""
+  endif
+
+  if line1 == line2
+    let expr = getline(line1)[col1-1 : col2-1]
+  else
+    let expr = getline(line1)[col1-1 : -1] . ' '
+          \ . join(getline(line1+1, line2-1), ' ')
+          \ . getline(line2)[0 : col2-1]
+  endif
+
+  " Calculate the position of cursor inside the expr
+  if line1 == line('.')
+    let p = col('.') - col1
+  else
+    let p = strlen(getline(line1)[col1-1 : -1])
+          \ + strlen(join(getline(line1 + 1, line('.') - 1), ' '))
+          \ + col('.')
+  endif
+
+  return strpart(expr, 0, p) . '__prefix__' . strpart(expr, p)
+endfunction
+
 function! fireplace#omnicomplete(findstart, base) abort
   if a:findstart
     let line = getline('.')[0 : col('.')-2]
@@ -116,7 +147,12 @@ function! fireplace#omnicomplete(findstart, base) abort
     try
 
       if fireplace#op_available('complete')
-        let response = fireplace#message({'op': 'complete', 'symbol': a:base, 'extra-metadata': ['arglists', 'doc']})
+        let response = fireplace#message({
+              \ 'op': 'complete',
+              \ 'symbol': a:base,
+              \ 'extra-metadata': ['arglists', 'doc'],
+              \ 'context': s:get_complete_context()
+              \ })
         let trans = '{"word": (v:val =~# ''[./]'' ? "" : matchstr(a:base, ''^.\+/'')) . v:val}'
         let value = get(response[0], 'value', get(response[0], 'completions'))
         if type(value) == type([])