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.lpp

@@ -0,0 +1,90 @@
+/*
+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 <cstdlib>
+#include "executable_semantics/syntax.tab.h"
+%}
+%option yylineno
+
+AND      "and"
+ARROW    "->"
+AUTO     "auto"
+BOOL     "Bool"
+BREAK    "break"
+CASE     "case"
+CHOICE   "choice"
+COMMENT  \/\/[^\n]*\n
+CONTINUE "continue"
+DBLARROW "=>"
+DEFAULT  "default"
+ELSE     "else"
+EQUAL    "=="
+FALSE    "false"
+FN       "fn"
+FNTY     "fnty"
+IF       "if"
+INT      "Int"
+MATCH    "match"
+NOT      "not"
+OR       "or"
+RETURN   "return"
+STRUCT   "struct"
+TRUE     "true"
+TYPE     "Type"
+VAR      "var"
+WHILE    "while"
+
+identifier    [A-Za-z_][A-Za-z0-9_]*
+integer_literal   [0-9]+
+
+%%
+
+{AND}      { return AND; }
+{ARROW}    { return ARROW; }
+{AUTO}     { return AUTO; }
+{BOOL}     { return BOOL; }
+{BREAK}    { return BREAK; }
+{CASE}     { return CASE; }
+{CHOICE}   { return CHOICE; }
+{COMMENT}  ;
+{CONTINUE} { return CONTINUE; }
+{DBLARROW} { return DBLARROW; }
+{DEFAULT}  { return DEFAULT; }
+{ELSE}     { return ELSE; }
+{EQUAL}    { return EQUAL; }
+{FALSE}    { return FALSE; }
+{FN}       { return FN; }
+{FNTY}     { return FNTY; }
+{IF}       { return IF; }
+{INT}      { return INT; }
+{MATCH}    { return MATCH; }
+{NOT}      { return NOT; }
+{OR}       { return OR; }
+{RETURN}   { return RETURN; }
+{STRUCT}   { return STRUCT; }
+{TRUE}     { return TRUE; }
+{TYPE}     { return TYPE; }
+{VAR}      { return VAR; }
+{WHILE}    { return WHILE; }
+
+{identifier} {
+  int n = strlen(yytext);
+  yylval.str = reinterpret_cast<char*>(malloc((n + 1) * sizeof(char)));
+  strncpy(yylval.str, yytext, n + 1);
+  return identifier;
+}
+{integer_literal} {
+  yylval.num = atof(yytext);
+  return integer_literal;
+}
+
+[ \t\n]+ ;
+. { return yytext[0]; }
+
+%%
+
+int yywrap()  { return 1; }