Adapting jsiek's executable semantics tooling for commit. (#237)

Notes versus what jsiek wrote:

- This adopts Bazel for building.
    - System-local versions of bison/flex are used. I found https://github.com/jmillikin/rules_bison, but those print a lot of warnings (things like -Wsign-compare IIRC) which makes builds hard to read. Plus I think the underlying bison_cc_library rule didn't work, so this would really only get a hermetic bison/flex build (helpful, but didn't seem worth more time).
    - I'm adding in a .bazeliskrc to push a somewhat more standard choice of bazel versions. I noticed I was getting unstable versions by default, possible Google-specific, but seemed good to include.
    - The `-lpthread` kludge.
- Turn all of the examples into golden tests.
    - Including adding a golden test rule.
- Fixed various style guide issues. For example:
    - Fixing function names to be CamelCase instead of snake_case (https://google.github.io/styleguide/cppguide.html#Function_Names)
    - Removed exception use (https://google.github.io/styleguide/cppguide.html#Exceptions)
    - File name fixes (https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/cpp_style_guide.md#file-names)
- Dropped `using` of `std` names -- I believe this is preferred (maybe we should be explicit about this in the Carbon style guide)
- Switched `enum` uses to `enum class` for ease-of-identification.
- Spent some time breaking out files to hopefully be easier to read/edit pieces, and understand relations between structs.
- Added `code requires` to `syntax.ypp` to address include issues

Possibly other things -- but the fundamental structure is, I believe, unchanged. I put in the golden tests pretty early to ensure I wasn't mutating output/results.

executable_semantics/ast/function_definition.cpp

@@ -0,0 +1,39 @@
+// 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 "executable_semantics/ast/function_definition.h"
+
+#include <iostream>
+
+namespace Carbon {
+
+auto MakeFunDef(int line_num, std::string name, Expression* ret_type,
+                Expression* param_pattern, Statement* body)
+    -> struct FunctionDefinition* {
+  auto* f = new struct FunctionDefinition();
+  f->line_num = line_num;
+  f->name = std::move(name);
+  f->return_type = ret_type;
+  f->param_pattern = param_pattern;
+  f->body = body;
+  return f;
+}
+
+void PrintFunDefDepth(struct FunctionDefinition* f, int depth) {
+  std::cout << "fn " << f->name << " ";
+  PrintExp(f->param_pattern);
+  std::cout << " -> ";
+  PrintExp(f->return_type);
+  if (f->body) {
+    std::cout << " {" << std::endl;
+    PrintStatement(f->body, depth);
+    std::cout << std::endl << "}" << std::endl;
+  } else {
+    std::cout << ";" << std::endl;
+  }
+}
+
+void PrintFunDef(struct FunctionDefinition* f) { PrintFunDefDepth(f, -1); }
+
+}  // namespace Carbon