Add standalone driver for fuzz targets

Useful for cases that don't need to use a fuzzing engine, but just run
the fuzz targets over some set of inputs, like for regression/CI tests.

Also added a POP3 fuzzer dictionary, seed corpus, and README with
examples.

src/fuzzers/standalone-driver.cc

@@ -0,0 +1,47 @@
+#include <cstdio>
+#include <cstdint>
+#include <cassert>
+#include <memory>
+#include <chrono>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size);
+extern "C" int LLVMFuzzerInitialize(int* argc, char*** argv);
+
+int main(int argc, char** argv)
+	{
+	using namespace std::chrono;
+	auto agg_start = high_resolution_clock::now();
+	auto num_inputs = argc - 1;
+	printf("Standalone fuzzer processing %d inputs\n", num_inputs);
+
+	LLVMFuzzerInitialize(&argc, &argv);
+
+	for ( auto i = 0; i < num_inputs; ++i )
+		{
+		auto input_file_name = argv[i + 1];
+		printf("  %s:", input_file_name);
+
+		auto f = fopen(input_file_name, "r");
+		assert(f);
+
+		fseek(f, 0, SEEK_END);
+		auto input_length = ftell(f);
+		fseek(f, 0, SEEK_SET);
+
+		auto input_buffer = std::make_unique<uint8_t[]>(input_length);
+		auto bytes_read = fread(input_buffer.get(), 1, input_length, f);
+		assert(bytes_read == input_length);
+
+		auto start = high_resolution_clock::now();
+		LLVMFuzzerTestOneInput(input_buffer.get(), input_length);
+		auto stop = high_resolution_clock::now();
+		auto dt = duration<double>(stop - start).count();
+
+		printf(" %6zu bytes, %f seconds\n", input_length, dt);
+		fclose(f);
+		}
+
+	auto agg_stop = high_resolution_clock::now();
+	auto agg_dt = duration<double>(agg_stop - agg_start).count();
+	printf("Processed %d inputs in %fs\n", num_inputs, agg_dt);
+}