Rename project from Lua to Lush

Binary is now 'lush', library is 'liblush.a'. Version banner shows
"Lush 0.1.0" with original Lua copyright preserved. Adds LUSH_VERSION
defines and _LUSH_VERSION global. Internal C API names, env vars, and
default paths are unchanged for upstream compatibility.

lapi.c

@@ -34,7 +34,8 @@
 
 const char lua_ident[] =
   "$LuaVersion: " LUA_COPYRIGHT " $"
-  "$LuaAuthors: " LUA_AUTHORS " $";
+  "$LuaAuthors: " LUA_AUTHORS " $"
+  "$LushVersion: " LUSH_RELEASE " $";