regex: remove From<regex_syntax::Error> impl

This removes a public `From` impl that automatically converts errors from
the regex-syntax crate to a regex::Error. This actually causes regex-syntax
to be a public dependency of regex, which was an oversight. We now remove
it, which completely breaks any source code coupling between regex and
regex-syntax.

See rust-lang#457

src/error.rs

@@ -11,8 +11,6 @@
 use std::fmt;
 use std::iter::repeat;
 
-use syntax;
-
 /// An error that occurred during parsing or compiling a regular expression.
 #[derive(Clone, PartialEq)]
 pub enum Error {
@@ -84,9 +82,3 @@ impl fmt::Debug for Error {
         }
     }
 }
-
-impl From<syntax::Error> for Error {
-    fn from(err: syntax::Error) -> Error {
-        Error::Syntax(err.to_string())
-    }
-}

src/exec.rs

@@ -228,7 +228,9 @@ impl ExecBuilder {
                     .allow_invalid_utf8(!self.only_utf8)
                     .nest_limit(self.options.nest_limit)
                     .build();
-            let expr = parser.parse(pat)?;
+            let expr = parser
+                .parse(pat)
+                .map_err(|e| Error::Syntax(e.to_string()))?;
             bytes = bytes || !expr.is_always_utf8();
 
             if !expr.is_anchored_start() && expr.is_any_anchored_start() {