use C89 escape for ESC

from git-svn-id: http://piumarta.com/svn2/peg/trunk@52 4d4069a9-7f2f-0410-87d9-cbc5f9740c0a
greg 0.4.5: enclose user actions in braces to allow local variables to be declared in C89

from git-svn-id: http://piumarta.com/svn2/peg/trunk@51 4d4069a9-7f2f-0410-87d9-cbc5f9740c0a
This is now matching peg-0.1.13