Rename parser dir to parse (#3178)

Continuing with #3070. Just a dir and file rename (mostly removing
prefixes, although for parse_tree_fuzzer and parse_tree_file_test I'm
dropping "tree" instead of "parse"). Everything in the parse dir should
be marked as a move.

toolchain/parse/parse_fuzzer.cpp

@@ -0,0 +1,44 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cstddef>
+#include <cstring>
+
+#include "llvm/ADT/StringRef.h"
+#include "toolchain/diagnostics/null_diagnostics.h"
+#include "toolchain/lexer/tokenized_buffer.h"
+#include "toolchain/parse/tree.h"
+
+namespace Carbon::Testing {
+
+// NOLINTNEXTLINE: Match the documented fuzzer entry point declaration style.
+extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
+                                      std::size_t size) {
+  // Ignore large inputs.
+  // TODO: See tokenized_buffer_fuzzer.cpp.
+  if (size > 100000) {
+    return 0;
+  }
+  static constexpr llvm::StringLiteral TestFileName = "test.carbon";
+  llvm::vfs::InMemoryFileSystem fs;
+  llvm::StringRef data_ref(reinterpret_cast<const char*>(data), size);
+  CARBON_CHECK(fs.addFile(
+      TestFileName, /*ModificationTime=*/0,
+      llvm::MemoryBuffer::getMemBuffer(data_ref, /*BufferName=*/TestFileName,
+                                       /*RequiresNullTerminator=*/false)));
+  auto source = SourceBuffer::CreateFromFile(fs, TestFileName);
+
+  // Lex the input.
+  auto tokens = Lex::TokenizedBuffer::Lex(*source, NullDiagnosticConsumer());
+  if (tokens.has_errors()) {
+    return 0;
+  }
+
+  // Now parse it into a tree. Note that parsing will (when asserts are enabled)
+  // walk the entire tree to verify it so we don't have to do that here.
+  Parse::Tree::Parse(tokens, NullDiagnosticConsumer(), /*vlog_stream=*/nullptr);
+  return 0;
+}
+
+}  // namespace Carbon::Testing