Move all lua related files to lua/ subfolder

Also remove the lexers sub directory from the Lua search path.
As a result we attempt to open fewer files during startup:

 $ strace -e open -o log ./vis +q config.h && wc -l log

In order to avoid having to modifiy all lexers which `require('lexer')`
we instead place a symlink in the top level directory.

 $ ./configure --disable-lua
 $ rm -rf lua

Should result in a source tree with most lua specifc functionality
removed.

1 files changed, 37 insertions, 0 deletions

diff --git a/lua/doc/README.md b/lua/doc/README.md
new file mode 100644
index 0000000..17531da
--- /dev/null
+++ b/lua/doc/README.md
@@ -0,0 +1,37 @@
+This is the destination directory for the LDoc documentation of Vis' Lua API
+as generated by the `make luadoc`.
+
+Unfortunately `ldoc(1)` seems to have a bug which generates broken reference
+links for custom types. The following patch seems to fix the issue:
+
+```
+From 96a1fbc8e972fedf665049a6351f46bc1aac1768 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Marc=20Andr=C3=A9=20Tanner?= <mat@brain-dump.org>
+Date: Tue, 6 Dec 2016 15:59:17 +0100
+Subject: [PATCH] Fix references to class types.
+
+For classes the generated HTML anchors need to be prefixed 'Class_'.
+---
+ ldoc/doc.lua | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/ldoc/doc.lua b/ldoc/doc.lua
+index e19b2df..f368b90 100644
+--- a/ldoc/doc.lua
++++ b/ldoc/doc.lua
+@@ -1073,9 +1073,9 @@ end
+ local function reference (s, mod_ref, item_ref)
+    local name = item_ref and item_ref.name or ''
+    -- this is deeply hacky; classes have 'Class ' prepended.
+---~    if item_ref and doc.class_tag(item_ref.type) then
+---~       name = 'Class_'..name
+---~    end
++       if item_ref and doc.class_tag(item_ref.type) then
++          name = 'Class_'..name
++       end
+    return {mod = mod_ref, name = name, label=s}
+ end
+ 
+-- 
+1.9.1
+```