Type: Add TypeManager->TypeList() and use for ListVal()

It turns out that for every ListVal we construct, we also allocate
and construct a new TypeList instance, even though they are all the
same. Pre-create and cache the type instances in a new TypeManager.

The following script runs ~10% faster for me after this change.

    global tbl: table[string] of string;
    global i = 0;
    while ( ++i < 10000000 )
        tbl["a"] = "a";

src/Type.h

@@ -2,6 +2,7 @@
 
 #pragma once
 
+#include <array>
 #include <list>
 #include <map>
 #include <optional>
@@ -866,6 +867,21 @@ protected:
 	TypePtr yield_type;
 	};
 
+// Holds pre-allocated Type objects.
+class TypeManager
+	{
+public:
+	TypeManager();
+
+	// Get a Typelist instance with the given type tag.
+	const TypeListPtr& TypeList(TypeTag t) const;
+
+private:
+	std::array<TypeListPtr, NUM_TYPES> base_list_types;
+	};
+
+extern TypeManager* type_mgr;
+
 // True if the two types are equivalent.  If is_init is true then the test is
 // done in the context of an initialization. If match_record_field_names is
 // true then for record types the field names have to match, too.