This attempts to fix issues ponylang#4130 and ponylang#4153.

The issue was when a private type in another package was used as a default value in a method call.

Fix to ponylang#4130
Since it has been decided to treat this as a bug instead of a missing error, this PR implements the fix suggested by @ergl, namely using `lookup_try()` instead of lookup() in call.c's `check_partial_function_call()` since it allows to permit private types.

Fix to ponylang#4153
This is also a simply fix to lookup.c that prevents a potential segfault by a dereferencing of `opt` (`typecheck_t* t = &opt->check;`) *before* `opt != NULL` was checked. As pointed out by @SeanTAllen, opt should not be NULL to begin with when lookup_nominal is called, and instead, an assert should be added and the NULL checks in that function removed.

…ult values"

This reverts commit 527864d.

This attempts to fix ponylang#4130

This crash stems from the use of a private type as defined in another package when it was used as a default value in a method call. Since it has been decided to treat this as a bug instead of a missing error, this PR implements the fix suggested by @ergl, namely using `lookup_try()` instead of lookup() in call.c's `check_partial_function_call()` since the former can be configured to permit private types.

Fix to ponylang#4153
This is a simply change to `lookup_nominal()` in lookup.c that prevents a potential segfault by a dereferencing of `opt` (`typecheck_t* t = &opt->check;`) *before* `opt != NULL` was checked. As pointed out by @SeanTAllen, opt should not be NULL to begin with when `lookup_nominal()` is called, and instead, an assert should be added and the NULL checks in that function removed.

With this PR, the two examples below that crashed the compiler now both compile:

Original example:
```pony
// inside the "useful" package

primitive _PrivateDefault

actor Useful[A: Any val]
  fun tag config(value: (A | _PrivateDefault) = _PrivateDefault): Useful[A] => this

// inside "main"

use "useful"

primitive This
primitive That

type Stuff is (This | That)

actor Main
  new create(env: Env) =>
    let u = Useful[Stuff].config()
```

Minimal example:
```pony
// In the "lib" pacakge

primitive _Private

primitive Public
  fun apply[T](v: (T | _Private) = _Private): None => None

// In main
use lib = "lib"

actor Main
  new create(env: Env) =>
    let p = lib.Public.apply[U8]()
    env.out.print(p.string())
```

Adding release notes

…unction/main.pony

Co-authored-by: Borja o'Cook <ergl@users.noreply.github.com>