Compile functions.zwc for each functions directory

instead of a big single `functions.zwc` file containing functions from
all the `functions` subdirectories.

This solution here better suits the functions autoloading mechanism in
zsh, as documented in
http://zsh.sourceforge.net/Doc/Release/Functions.html#index-autoloading-functions

Given `~/.zim/modules/foo/functions` is in the `fpath`, autoload looks
for functions in the following files, picking the **newest one** in this
order:
* `~/.zim/modules/foo/functions.zwc`, which should contain all
  functions in the directory named `functions`
* `~/.zim/modules/foo/functions/function.zwc`
* `~/.zim/modules/foo/functions/function`

With this approach, we're also having individual entries back in `fpath`
for the `functions` subdirectory of each enabled module (as was the case
before commit 55df5a4).

This is also probably closest to the original plan at #86. The current
code was not compiling "*all* files in a folder into a single digest",
but instead compiling *all* files in *all* the functions folders into a
single digest, which was a bad idea. This commit fixes this.

Fixes #231. Fixes #232. Closes #234.

Closes #239

init.zsh

@@ -20,7 +20,7 @@ fi
   setopt LOCAL_OPTIONS EXTENDED_GLOB
 
   # autoload searches fpath for function locations; add enabled module function paths
-  fpath=(${ZIM_HOME}/functions.zwc ${ZIM_HOME}/modules/prompt/functions ${fpath})
+  fpath=(${ZIM_HOME}/modules/${^zmodules}/functions(/FN) ${fpath})
 
   for mod_function in ${ZIM_HOME}/modules/${^zmodules}/functions/^([_.]*|prompt_*_setup|README*|*.zwc|*.zwc.old)(-.N:t); do
     autoload -Uz ${mod_function}

login_init.zsh

@@ -8,8 +8,7 @@
 #
 
 (
-  local file
-  local zmodule
+  local dir file
   setopt LOCAL_OPTIONS EXTENDED_GLOB
   autoload -U zrecompile
 
@@ -20,11 +19,13 @@
   zrecompile -pq ${ZDOTDIR:-${HOME}}/.zshrc
 
   # zcompile enabled module autoloaded functions
-  zrecompile -pq ${ZIM_HOME}/functions ${ZIM_HOME}/modules/${^zmodules}/functions/^([_.]*|prompt_*_setup|README*|*.zwc|*.zwc.old)(-.N)
+  for dir in ${ZIM_HOME}/modules/${^zmodules}/functions(/FN); do
+    zrecompile -pq ${dir}.zwc ${dir}/^([_.]*|prompt_*_setup|README*|*.zwc|*.zwc.old)(-.N)
+  done
 
   # zcompile enabled module init scripts
-  for zmodule (${zmodules}); do
-    zrecompile -pq ${ZIM_HOME}/modules/${zmodule}/init.zsh
+  for file in ${ZIM_HOME}/modules/${^zmodules}/init.zsh(-.N); do
+    zrecompile -pq ${file}
   done
 
   # zcompile all prompt setup scripts