HE built-in `parse` is not supported

## What is your problem?

The HE built-in `parse` that allows to take a string representing a MeTTa term and instantiating it into a MeTTa term, is not supported.

## How to reproduce your problem?

Run the following program with PeTTa:

```scheme
!(parse "(R A B)")
```

## What would you normally expect?

```
(R A B)
```

## What do you get instead?

```
(parse (R A B))
```
