Let immutable map puts widen on concrete classes

[delacry]

Stacked on #56, so merge that one first. After it's merged, I'll rebase this onto 0.2.x and retarget it.

This fixes the same problem as #56, but for maps. On a concrete map class, some trait docblocks didn't match the map interfaces:

/** @param ImmutableHashMap<string, int> $map */
$map->put($id, $flag);  // int $id, bool $flag
// before: PHPStan error
// now:    ImmutableMap<int|string, bool|int>, like the interface

$map->containsKey($id);
// before: PHPStan error
// now:    fine, every map interface accepts any key

$map->toArray();
// before: array<int>
// now:    array<string, int>

It also fixes a runtime bug: intMapOf([1 => 'a'])->putIfAbsent('b', 'x') threw InvalidKeyTypeException, while put() falls back to a hash map. putIfAbsent() on immutable IntMap and StringMap now falls back the same way.

Docs:

• The FAQ, the map guide and the class docblocks said every foreign key throws. Only the mutable IntMap and StringMap throw.
• The extending guide shows how a map class that rebuilds itself can keep put() strict with @method.
• UPGRADE-0.2.md is updated.

The new tests/Type/Map/ConcreteClassTypeTest.php and the putIfAbsent_rejects_* tests fail without this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!