[RFC] Add assert statement to make unit testing easier for Poke code

[Mohammad-Reza Nabipoor]

Hi.

== Proposal

I suggest to support the following `assert` statements:

    assert (condition);
    assert (condition, message);
    assert condition;
    assert condition, message;

The parser will replace this statement with the following function call:

    _pkl_assert (condition, message, lineinfo)

The `lineinfo` is a string containing the location of the statement:

    "<filename>:<line>:<col>"

And implementation of `_pkl_assert` will something like this in `pkl-rt.pk`:

```poke
fun _pkl_assert = (uint<1> condition, string msg, string lineinfo)
  {
    if (cond)
      return;

    raise Exception {
      code = EC_assert,
      msg = "Assertion failed at " + lineinfo + (msg'length ? ": " + msg : ""),
      exit_status = 1,
    };
  }
```


== Usage

The main use of `assert` in unit testing of Poke codes.

For example, this is the re-write of some of string tests in
`testsuite/poke.pkl/`:

```poke
type TestFn = () void;
type Test = struct
  {
    string name;
    TestFn func;  /* Currently doesn't work! error: invalid type in struct 
field */
  };

var tests = [
  Test {
    name = "index",
    func = lambda void:
      {
        var n = 0;

        assert ("foo"[n] == 102UB);
        assert ("foo"[0] == 102UB);

        var s = "bar";

        n = 3;
        assert (("foo" + s)[n] == 98UB);
        assert (("foo" + "bar")[3] == 98UB);
      }
  },
  Test {
    name = "escape sequence",
    func = lambda void:
      {
        var s = "\1\12\1234;\12==\n";

        assert (s'length == 0o11UL);
        assert (s[0] == 0o1UB);
        assert (s[1] == 0o12UB);
        assert (s[2] == 0o123UB);
        assert (s[3:] == "4;\n==\n");

        assert ("\\"[0] == 0x5c);
      }
  },
];

for (t in tests)
  {
    try t.func();
    catch (Exception ex)
      {
        printf ("FAIL %s: %s\n", t.name, ex.msg);
        continue;
      }
    print ("PASS " + t.name + "\n");
  }
```


We also can define a pickle called `unittest` test to add some types and 
functions
to make writing unit tests easier.

I think this is very useful Poke users. And very easy to implement.
The only thing `assert` give us is the location of the statement.
WDYT?
Any suggestion?


Regards,
Mohammad-Reza