ast, checker: improved accessing map value that contain pointers requires an or {} block

[shove70]

1. Finish any remaining work: checker: remove unnecessary struct ref field initialization checks and notifications at map initializing(fix #20245) #20251 (comment)
2. Recursively checks whether the map value contains a pointer.
3. Add tests.

struct Struct {
	field &int
}

fn main() {
	m := map[string][]Struct{}
	a := m['key']
	println(a)
}

outputs:

a.v:7:8: notice: accessing map value that contain pointers requires an `or {}` block outside `unsafe`
    5 | fn main() {
    6 |     m := map[string][]Struct{}
    7 |     a := m['key']
      |           ~~~~~~~
    8 |     println(a)
    9 | }
[]

[spytheman]

Excellent work!