Revise general explanation on type casting

[jaehyun1ee]

Following the discussion made at #1351, this PR revises the general explanation on type casting given in section 8.11. Casts* to the following:

;; Before

P4 provides a limited set of casts between types. A cast is written
`(t) e`, where `t` is a type and `e` is an expression. Casts are only
permitted on base types and derived types introduced by `typedef`, `type`, and `enum`.
While this design is arguably more onerous for programmers, it has several benefits:

;; After

P4 provides a limited set of casts between types. A cast is written
`(t) e`, where `t` is a type and `e` is an expression. Casts are only permitted in
cases as defined in section <<sec-explicit-casts>> for explicit casts and
section <<sec-implicit-casts>> for implicit casts. While this design is arguably
more onerous for programmers, it has several benefits:

Rather than stating "Casts are only permitted on base types and derived types introduced by typedef, type, and enum." (which is quite inaccurate), it puts forward references to the following subsections. Because:

• Not all base types can be casted (e.g., void, error, match_kind, and string)
• Derived types, such as struct and header can be casted, e.g., at initialization.

[jafingerhut]

At 2025-Jan-13 vlstill agreed to review in detail. No other LDWG members could think of reasons to object based on our discussion, but they should review and add their comments if they wish.

[vlstill]

I agree with the new wording. For the sake of readability of this area of the spec I suggest some tweaks to the following text.

[vlstill]

I think the last sentence makes little less sense with the forward references. I suggest something like this:

Suggested change

	section <<sec-implicit-casts>> for implicit casts. While this design is arguably
	more onerous for programmers, it has several benefits:
	section <<sec-implicit-casts>> for implicit casts. Main goals of this design are:

and possibly add

• It limits implicit casts to the bare minimum.

[jaehyun1ee]

Thank you for the suggestion, I revised the sentences as:

Casts in P4 are permitted only under the conditions specified in section sec-explicit-casts for explicit casts and section sec-implicit-casts for implicit casts. By allowing casts only in limited scenarios, this restriction is more onerous for programmers, but it also provides several benefits.

I tried to make it explicit that casts are very limited in P4.

[vlstill]

nitpick: maybe this would read more naturally:

Suggested change

	By allowing casts only in limited scenarios, this restriction is more
	onerous for programmers, but it also provides several benefits.
	Allowing casts only in limited scenarios is more onerous for
	programmers, but it also provides several benefits.