Add protobuf descriptor and compiler plugin support

Implement accessors and builders for FileDescriptorSet,
CodeGeneratorRequest, CodeGeneratorResponse, and related types.

src/lib.rs

@@ -1,3 +1,4 @@
+pub mod proto_gen;
 use std::fmt;
 
 #[derive(Debug, PartialEq, Eq)]
@@ -180,7 +181,7 @@ impl<'a> ProtoAccessor<'a> {
     }
 
     /// Returns the value and wire type of the last occurrence of the specified field.
-    pub fn get_value(&self, field_number: u32) -> Result<(&[u8], WireType)> {
+    pub fn get_value(&self, field_number: u32) -> Result<(&'a [u8], WireType)> {
         let mut last_value = None;
         for item in self.fields() {
             let (tag, value) = item?;