Allow operators to use '\'

Fix #246

src/Cryptol/Parser/Lexer.x

@@ -31,7 +31,7 @@ $unitick        = \x7
 @id_next      = [a-zA-Z0-9_'] | $unilower | $uniupper | $unidigit | $unitick
 
 @id           = @id_first @id_next*
-@op           = ([\!\@\#\$\%\^\&\*\~\>\<\?\+\=\|\/] | $unisymbol)+
+@op           = ([\!\@\#\$\%\^\&\*\~\>\<\?\+\=\|\/\\] | $unisymbol)+
 
 @qual         = (@id $white* :: $white*)+
 @qual_id      = @qual @id

tests/parser/infix-3.cry

@@ -0,0 +1,3 @@
+
+(/\) : Bool -> Bool -> Bool
+p /\ q = p && q

tests/parser/infix-3.icry

@@ -0,0 +1,2 @@
+:l infix-3.cry
+:t (/\)

tests/parser/infix-3.icry.stdout

@@ -0,0 +1,4 @@
+Loading module Cryptol
+Loading module Cryptol
+Loading module Main
+(/\) : Bool -> Bool -> Bool