Add map functions for Error<> and Info<> ranges. (Marwes#86)

It's possible to `map_err_range` for `ParseResult<>` too, but it's
awkward because the output type needs to be a compatible `StreamOnce`.
As suggested in
Marwes#86 (comment),
it's probably best to either change the parse result type entirely, or
wait for rust-lang/rust#21903.

This at least helps consumers convert `ParseError<>` into something
that can implement `std::fmt::Display`.

src/primitives.rs

@@ -56,6 +56,18 @@ pub enum Info<T, R> {
     Borrowed(&'static str),
 }
 
+impl<T, R> Info<T, R> {
+    pub fn map_range<F, S>(self, f: F) -> Info<T, S> where F: FnOnce(R) -> S {
+        use self::Info::*;
+        match self {
+            Token(t) => Token(t),
+            Range(r) => Range(f(r)),
+            Owned(s) => Owned(s),
+            Borrowed(x) => Borrowed(x),
+        }
+    }
+}
+
 impl<T: PartialEq, R: PartialEq> PartialEq for Info<T, R> {
     fn eq(&self, other: &Info<T, R>) -> bool {
         match (self, other) {
@@ -110,6 +122,18 @@ pub enum Error<T, R> {
     Other(Box<StdError + Send + Sync>),
 }
 
+impl<T, R> Error<T, R> {
+    pub fn map_err_range<F, S>(self, f: F) -> Error<T, S> where F: FnOnce(R) -> S {
+        use self::Error::*;
+        match self {
+            Unexpected(x) => Unexpected(x.map_range(f)),
+            Expected(x) => Expected(x.map_range(f)),
+            Message(x) => Message(x.map_range(f)),
+            Other(x) => Other(x),
+        }
+    }
+}
+
 impl<T: PartialEq, R: PartialEq> PartialEq for Error<T, R> {
     fn eq(&self, other: &Error<T, R>) -> bool {
         match (self, other) {