feat(json): add a json library

This is a wrapper around any of the following Lua modules; 'lua-cjson',
'dkjson', or 'rapidjson'.

src/pegasus/json.lua

@@ -0,0 +1,62 @@
+-- Wrapper library that supports multiple JSON libraries
+local found, jsonlib
+
+found, jsonlib = pcall(require, "cjson.safe") -- lua-cjson
+if found then
+  return setmetatable({
+    -- decode is available
+    -- encode is available
+    -- null is available
+    available = true,
+    makeArray= function(t)
+      assert(type(t) == "table", "expected a table")
+      return setmetatable(t, jsonlib.array_mt)
+    end,
+  }, { __index = jsonlib })
+end
+
+
+found, jsonlib = pcall(require, "dkjson") -- dkjson
+if found then
+  local array_mt = { __jsontype='array'}
+  return setmetatable({
+    -- decode is available
+    -- encode is available
+    -- null is available
+    available = true,
+    makeArray= function(t)
+      assert(type(t) == "table", "expected a table")
+      return setmetatable(t, array_mt)
+    end,
+  }, { __index = jsonlib })
+end
+
+
+found, jsonlib = pcall(require, "rapidjson") -- rapidjson
+if found then
+  local array_mt = { __jsontype='array'}
+  return setmetatable({
+    decode = function(value)
+      return jsonlib.decode(value, 1, jsonlib.null)
+    end,
+    -- encode is available
+    -- null is available
+    available = true,
+    makeArray= function(t)
+      assert(type(t) == "table", "expected a table")
+      return setmetatable(t, array_mt)
+    end,
+  }, { __index = jsonlib })
+end
+
+
+local err = function()
+  return nil, "no json library available, install 'lua-cjson', 'dkjson', or 'rapidjson'"
+end
+
+return {
+  available = false,
+  encode = err,
+  decode = err,
+  makeArray = err,
+}