Change remaining uses of Bool to bool, following #750. (#1901)

The most significant change here is that explorer now uses the chosen spelling
rather than the old `Bool` spelling. Also update a few documentation examples
and some skeletal design docs to use the chosen spelling.

explorer/ast/expression.cpp

@@ -288,7 +288,7 @@ void Expression::PrintID(llvm::raw_ostream& out) const {
       out << (cast<BoolLiteral>(*this).value() ? "true" : "false");
       break;
     case ExpressionKind::BoolTypeLiteral:
-      out << "Bool";
+      out << "bool";
       break;
     case ExpressionKind::IntTypeLiteral:
       out << "i32";