Change record field anonymous functions to use lambda expressions

There was an alternate syntax to assign anonymous functions to record
fields that was never migrated to use the new lambda expression
machinery (and so didn't allow referencing variables in outer scope):

    type myrec: record {
        foo: function(a: string);
    };

    local o = "o";
    local mr = myrec($foo(a: string) = { print a + o; });

testing/btest/language/lambda-record-field.zeek

@@ -0,0 +1,13 @@
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+type myrec: record {
+	foo: function(a: string);
+};
+
+event zeek_init()
+	{
+	local w = "world";
+	local mr = myrec($foo(a: string) = { print a + w; });
+	mr$foo("hello");
+	}