Fix floating point separators in Rust lexer (#1581)

The Rust lexer does not currently support the use of `_` in floating
point numbers. If the separator comes immedately after the first digit,
Rouge lexes it as an error. This commit fixes that bug.

lib/rouge/lexers/rust.rb

@@ -167,7 +167,7 @@ def macro_closed?
         flt = /f32|f64/
 
         rule %r(
-          [0-9]+
+          [0-9_]+
           (#{dot}  #{exp}? #{flt}?
           |#{dot}? #{exp}  #{flt}?
           |#{dot}? #{exp}? #{flt}

spec/visual/samples/rust

@@ -400,6 +400,10 @@ fn int_literals_delimiter() {
     let red_color = 0xff_60_60;
 }
 
+fn float_literals_delimiter() {
+    let billion = 1000_000_000.0;
+}
+
 // Some hidden lines by starting with hash
 # extern crate core;
 # use core::str;