Support recent versions of OCaml, which default to -safe-string

[yallop]

With OCaml 4.07.1, compilation fails as follows:

File "location.ml", line 47, characters 15-38:
Error: This expression has type bytes but an expression was expected of type
         string

[bb010g]

Note that this is fixed in the cleanup branch by cb0fcc34bace0f7a62446c7483f9000f4b4c47db.