Replace protobuf with flatbuffer

Roadmap.md

@@ -132,7 +132,7 @@ const char* deno_last_exception(Deno* d);
 
 There are three layers of API to consider:
 * L1: the low-level message passing API exported by libdeno (L1),
-* L2: the protobuf messages used internally (L2),
+* L2: the flatbuffer messages used internally (L2),
 * L3: the final "deno" namespace exported to users (L3).
 
 ### L1
@@ -176,7 +176,7 @@ function main() {
 
 ### L2
 
-https://github.com/denoland/deno/blob/master/msg.proto
+https://github.com/denoland/deno/blob/master/src/msg.fbs
 
 ### L3