Replace nonstd::optional by C++17 std::optional

Removes third-party code that is no longer necessary now that we use C++
17 as build standard. Future uses of optionalt should instead use
std::optional directly.

Author: tautschnig

CODEOWNERS

@@ -18,7 +18,6 @@
 /src/json-symtab-language/ @martin-cs
 /src/langapi/ @kroening @tautschnig @peterschrammel
 /src/xmllang/ @kroening @tautschnig @peterschrammel
-/src/nonstd/ @peterschrammel
 /src/solvers/flattening @martin-cs @kroening @tautschnig @peterschrammel
 /src/solvers/floatbv @martin-cs @kroening
 /src/solvers/miniBDD @tautschnig @kroening

doc/architectural/folder-walkthrough.md

@@ -50,7 +50,6 @@ containing the code for a different part of the system.
   * \ref xmllang
   * \ref util
   * \ref miniz
-  * \ref nonstd
 
 In the top level of `src` there are only a few files:
 
@@ -166,7 +165,6 @@ digraph directory_dependencies {
     big_int [label = "big-int", URL = "\ref big-int"];
     miniz [URL = "\ref miniz"];
     util [URL = "\ref util"];
-    nonstd [URL = "\ref nonstd"];
     json [URL = "\ref json"];
     xmllang [URL = "\ref xmllang"];
     assembler [URL = "\ref assembler"];
@@ -202,6 +200,5 @@ digraph directory_dependencies {
   xmllang -> util;
   assembler -> util;
   util -> big_int;
-  util -> nonstd;
 }
 \enddot

jbmc/src/java_bytecode/ci_lazy_methods.h

@@ -81,7 +81,7 @@ class method_bytecodet
   {
     const auto it = map.find(method_id);
     if(it == map.end())
-      return opt_reft();
+      return std::nullopt;
     return std::cref(it->second);
   }
 };