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Default values for function parameters, closes #368

This PR adds the ability to specify default values for function
arguments.

Optional parameters need to come right after mandatory ones:

```
⧐  f test(x = 1, y){}
 parser errors:
	found mandatory parameter after optional one
	[1:15]	f test(x = 1, y){}
```

and they can use any kind of expression:

```
⧐  f test(x = 1){ x }; test()
1
⧐  f test(x = []){ x }; test()
[]
⧐  f test(x = {}){ x }; test()
{}
⧐  f test(x = 1..10){ x }; test()
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
```

If a mandatory argument is not passed, ABS will now give you a more
detailed message:

```
⧐  f test(x){ x }; test()
ERROR: argument x to function f test(x) {x} is missing, and doesn't have a default value
	[1:1]	f test(x){ x }; test()
```

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