Move binpac code into the main Zeek repository

This is based on commit 48f75b5f6415fe9d597e3e991cec635b1bc400dc from
the binpac repository.

tools/binpac/src/pac_enum.cc

@@ -0,0 +1,58 @@
+#include "pac_enum.h"
+
+#include "pac_exception.h"
+#include "pac_expr.h"
+#include "pac_exttype.h"
+#include "pac_output.h"
+#include "pac_typedecl.h"
+
+Enum::Enum(ID* id, Expr* expr) : id_(id), expr_(expr) {}
+
+Enum::~Enum() {
+    delete id_;
+    delete expr_;
+}
+
+void Enum::GenHeader(Output* out_h, int* pval) {
+    ASSERT(pval);
+    if ( expr_ ) {
+        if ( ! expr_->ConstFold(global_env(), pval) )
+            throw ExceptionNonConstExpr(expr_);
+        out_h->println("%s = %d,", id_->Name(), *pval);
+    }
+    else
+        out_h->println("%s,", id_->Name());
+    global_env()->AddConstID(id_, *pval);
+}
+
+EnumDecl::EnumDecl(ID* id, EnumList* enumlist) : Decl(id, ENUM), enumlist_(enumlist) {
+    ID* type_id = id->clone();
+    datatype_ = new ExternType(type_id, ExternType::NUMBER);
+    extern_typedecl_ = new TypeDecl(type_id, nullptr, datatype_);
+}
+
+EnumDecl::~EnumDecl() {
+    delete_list(EnumList, enumlist_);
+    delete extern_typedecl_;
+}
+
+void EnumDecl::Prepare() {
+    // Do nothing
+}
+
+void EnumDecl::GenForwardDeclaration(Output* out_h) {
+    out_h->println("// NOLINTNEXTLINE(performance-enum-size)");
+    out_h->println("enum %s {", id_->Name());
+    out_h->inc_indent();
+    int c = 0;
+    foreach (i, EnumList, enumlist_) {
+        (*i)->GenHeader(out_h, &c);
+        ++c;
+    }
+    out_h->dec_indent();
+    out_h->println("};");
+}
+
+void EnumDecl::GenCode(Output* out_h, Output* /* out_cc */) {
+    // Do nothing
+}