bidi.c: update the API.

The input length field is now a size_t rather than an int, on general
principles. The return value is now void (we weren't using the
previous return value at all). And we now require the client to have
previously allocated a BidiContext, which will allow allocated storage
to be reused between runs, saving a lot of churn on malloc.

(However, the current BidiContext doesn't contain anything
interesting. I could have moved the existing mallocs into it, but
there's no point, since I'm about to rewrite the whole thing anyway.)

defs.h

@@ -170,6 +170,8 @@ typedef struct SessionSpecial SessionSpecial;
 
 typedef struct StripCtrlChars StripCtrlChars;
 
+typedef struct BidiContext BidiContext;
+
 /*
  * A small structure wrapping up a (pointer, length) pair so that it
  * can be conveniently passed to or from a function.