Allow underscores in hex strings.

test/libsolidity/syntaxTests/literals/hex_string_duplicate_underscore.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f() public pure {
+        hex"12__34";
+    }
+}
+// ----
+// ParserError: (52-60): Invalid use of number separator '_'.

test/libsolidity/syntaxTests/literals/hex_string_leading_underscore.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f() public pure {
+        hex"_1234";
+    }
+}
+// ----
+// ParserError: (52-57): Invalid use of number separator '_'.

test/libsolidity/syntaxTests/literals/hex_string_misaligned_underscore.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f() public pure {
+        hex"1_234";
+    }
+}
+// ----
+// ParserError: (52-56): Expected even number of hex-nibbles.

test/libsolidity/syntaxTests/literals/hex_string_trailing_underscore.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f() public pure {
+        hex"1234_";
+    }
+}
+// ----
+// ParserError: (52-61): Invalid use of number separator '_'.

test/libsolidity/syntaxTests/literals/hex_string_underscores_valid.sol

@@ -0,0 +1,3 @@
+contract C {
+    bytes constant c = hex"12_3456_789012";
+}