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/syntax_helpers.cpp

@@ -0,0 +1,45 @@
+// 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/syntax_helpers.h"
+
+#include <iostream>
+
+#include "executable_semantics/interpreter/interpreter.h"
+#include "executable_semantics/interpreter/typecheck.h"
+
+namespace Carbon {
+
+char* input_filename = nullptr;
+
+void PrintSyntaxError(char* error, int line_num) {
+  std::cerr << input_filename << ":" << line_num << ": " << error << std::endl;
+  exit(-1);
+}
+
+void ExecProgram(std::list<Declaration*>* fs) {
+  std::cout << "********** source program **********" << std::endl;
+  for (const auto& decl : *fs) {
+    PrintDecl(decl);
+  }
+  std::cout << "********** type checking **********" << std::endl;
+  state = new State();  // Compile-time state.
+  std::pair<TypeEnv*, Env*> p = TopLevel(fs);
+  TypeEnv* top = p.first;
+  Env* ct_top = p.second;
+  std::list<Declaration*> new_decls;
+  for (const auto& i : *fs) {
+    new_decls.push_back(TypeCheckDecl(i, top, ct_top));
+  }
+  std::cout << std::endl;
+  std::cout << "********** type checking complete **********" << std::endl;
+  for (const auto& decl : new_decls) {
+    PrintDecl(decl);
+  }
+  std::cout << "********** starting execution **********" << std::endl;
+  int result = InterpProgram(&new_decls);
+  std::cout << "result: " << result << std::endl;
+}
+
+}  // namespace Carbon