THRIFT-2513 clean up enum value assignment
Patch: Dave Watson

This closes #88

Summary:
Clean up how enum values are handled if an integer value is not
explicitly specified in the thrift file.

For example, the following used to be a compile error, but
works now:

  enum MyEnum {
	SOMEVALUE
  }
  struct MyStruct {
	1: MyEnum e = SOMEVALUE
  }

This change also cleans up some of the error handling with out-of-range
values.  Previously thrift simply issued a warning for enum values that
didn't fit in an i32, but serialized them as i32 anyway.  Now
out-of-range enum values result in a compile failure.

Test Plan:
Included a new unit test to verify the assignment of enum values.  I
also verified that g++ makes the same enum value assignments when
compiling these enums as C++ code.
diff --git a/test/EnumTest.thrift b/test/EnumTest.thrift
new file mode 100644
index 0000000..e6376f5
--- /dev/null
+++ b/test/EnumTest.thrift
@@ -0,0 +1,70 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ *
+ * Contains some contributions under the Thrift Software License.
+ * Please see doc/old-thrift-license.txt in the Thrift distribution for
+ * details.
+ */
+
+enum MyEnum1 {
+  ME1_0 = 0,
+  ME1_1 = 1,
+  ME1_2,
+  ME1_3,
+  ME1_5 = 5,
+  ME1_6,
+}
+
+enum MyEnum2 {
+  ME2_0,
+  ME2_1,
+  ME2_2,
+}
+
+enum MyEnum3 {
+  ME3_0,
+  ME3_1,
+  ME3_N2 = -2,
+  ME3_N1,
+  ME3_D0,
+  ME3_D1,
+  ME3_9 = 9,
+  ME3_10,
+}
+
+enum MyEnum4 {
+  ME4_A = 0x7ffffffd
+  ME4_B
+  ME4_C
+  // attempting to define another enum value here fails
+  // with an overflow error, as we overflow values that can be
+  // represented with an i32.
+}
+
+enum MyEnum5 {
+  // attempting to explicitly use values out of the i32 range will also fail
+  // ME5_A = 0x80000000,
+  // ME5_B = 0x100000000,
+}
+
+struct MyStruct {
+  1: MyEnum2 me2_2 = MyEnum1.ME2_2
+  2: MyEnum3 me3_n2 = MyEnum3.ME3_N2
+  3: MyEnum3 me3_d1 = MyEnum3.ME3_D1
+}
+