support capitalized Boolean values

in the Frenetic project, we recently discovered that we had been using 'False' instead of 'false' in our OASIS files [1]. this is rather unfortunate because 'False' is interpreted as true. :-/

this pull request adds explicit support for 'False' and 'FALSE' to be interpreted as `EBool false`, with corresponding support for 'True' and 'TRUE' as well.

hopefully this will prevent others' naive mistakes in the future!


thank you for your consideration,
Andrew


[1] https://github.com/frenetic-lang/ocaml-openflow/pull/59