sys: support type aliases (aka typedefs)

Complex types that are often repeated can be given short type aliases using the
following syntax:

```
type identifier underlying_type
```

For example:

```
type signalno int32[0:65]
type net_port proc[20000, 4, int16be]
```

Then, type alias can be used instead of the underlying type in any contexts.
Underlying type needs to be described as if it's a struct field, that is,
with the base type if it's required. However, type alias can be used as syscall
arguments as well. Underlying types are currently restricted to integer types,
`ptr`, `ptr64`, `const`, `flags` and `proc` types.

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