Move Meta to a separate module.

Also cleaned up some of the code around loading the core module.

test/meta/eval_compile_error.wren

@@ -1 +1,3 @@
+import "meta" for Meta
+
 Meta.eval("!!") // expect runtime error: Could not compile source code.

test/meta/eval_existing_scoped_variable.wren

@@ -1,3 +1,5 @@
+import "meta" for Meta
+
 var y
 
 Meta.eval("y = 2")

test/meta/eval_not_string.wren

@@ -1 +1,3 @@
+import "meta" for Meta
+
 Meta.eval(123) // expect runtime error: Source code must be a string.