Do cleanup on explorer fuzzing infrastructure. (#2790)

I'm partly doing this because the current setup would be difficult to share with the toolchain. e.g., ProtoToCarbon isn't explorer-specific, but the only way to run it via CLI is the explorer's fuzzverter. I want a separate tool.

This change:

- Adds a //common/fuzzing:proto_to_carbon tool.
  - The rest of fuzzverter is now just //explorer/fuzzing:ast_to_proto.
  - The change simplifies overall handling and removes a LLVM CLI dependency.
- Stops allowing unknown fields in the proto.
  - This has mostly led to forgetting to remove fuzzer inputs that were for removed features.
- Moves more non-explorer-specific bits to //common/fuzzing.
- Cleans up remaining pieces in //explorer/fuzzing
  - Merges the //explorer/fuzzing proto tests, which deduplicates AstToString copies.
    - These tests also had duplicate dependencies, etc -- and all complete in ~6s.
  - Updates and fixes regen_corpus which was previously broken by other changes.
  - Updates the README to reflect changes.
- Removes obsolete proto-fuzzer build configuration (AFAICT this is no longer needed).

explorer/fuzzing/explorer_fuzzer.cpp

@@ -10,5 +10,5 @@
 
 DEFINE_TEXT_PROTO_FUZZER(const Carbon::Fuzzing::Carbon& input) {
   // Only verifying it doesn't crash.
-  (void)Carbon::ParseAndExecute(input.compilation_unit());
+  (void)Carbon::ParseAndExecute(input);
 }