Add Steve Johnson-style parser recovery rules:

SourceUnit = Error $
    Block = '{' Error '}'
    ContractDefinition = '{' Error '}'
    Statement = Error ';'

Co-Authored-By: chriseth <chris@ethereum.org>

test/libsolidity/errorRecoveryTests/constructor_recovers.sol

@@ -0,0 +1,20 @@
+pragma solidity >=0.0.0;
+
+contract Error1 {
+  constructor() public {
+    balances[tx.origin] = ; // missing RHS.
+  }
+
+  // Without error recovery we stop due to the above error.
+  // Error recovery however recovers at the above ';'
+  // There should be an AST for the above, albeit with error
+  // nodes.
+
+  // This function parses properly and should give AST info.
+  function five() public view returns(uint) {
+    return 5;
+  }
+}
+// ----
+// ParserError: (95-96): Expected primary expression.
+// Warning: (95-96): Recovered in Statement at ';'.