lang: Update map type definition to include a prefix

It turns out that some planned additions to the parser make it so that
the map type definition can be ambiguous. As a result, this patch
updates the definition so that the map definition is not confused with
an open curly bracket anywhere.

Thanks to pestle and stbenjamin for their help understanding yacc!

test/test-examples.sh

@@ -11,6 +11,8 @@ cd "${ROOT}"
 
 failures=''
 
+# TODO: test examples/lang/ directory to see if the .mcl files compile correctly
+
 buildout='test-examples.out'
 # make symlink to outside of package
 linkto="`pwd`/examples/lib/"