Fix escape sequences in Rust lexer

As discussed in rouge-ruby#1075, escaped double quotes were not being interpreted
correctly. This was due to an escaped double quote not being included in
the escape sequence regex in the Rust lexer.

This commit adds the double quote and the numeral zero (the null
character). See:
https://static.rust-lang.org/doc/master/reference.html#characters-and-strings

This fixes rouge-ruby#1075.

lib/rouge/lexers/rust.rb

@@ -56,7 +56,7 @@ def macro_closed?
       id = /[a-z_]\w*/i
       hex = /[0-9a-f]/i
       escapes = %r(
-        \\ ([nrt'\\] | x#{hex}{2} | u#{hex}{4} | U#{hex}{8})
+        \\ ([nrt'"\\0] | x#{hex}{2} | u#{hex}{4} | U#{hex}{8})
       )x
       size = /8|16|32|64/
 

spec/visual/samples/rust

@@ -18,6 +18,11 @@ fn main() {
 
 let mut f = File::open("username.txt")?;
 
+println!("a\\");
+println!("a\n");
+println!("a\"b");
+println!("a\'");
+
 debug!("test %?", a.b);
 debug!("test %u", a.c);
 debug!("test %i", a.d);