[embind] Add pointer policies for setting val object properties.

[brendandahl]

After #24175, val required a pointer policy when creating a val with a pointer. In val.set(key, value), a temporary val object is created, but there was no way to set the policy from user code. This patch allows the user to pass in allow_raw_pointers() to enable this.

[brendandahl]

an aside: @RReverser do you have any thoughts on how to add policies to to val's call and operator(). Unfortunately, they both take variadic arguments already, so I don't see a great way to add on a Policy.... I was thinking either

1. Make a wrapper struct/tuple thing for Policy arguments
2. Try to separate the arguments and filter the Policy arguments into a list, but this looks rather painful to do in c++11.

[RReverser]

@brendandahl Can't think of a good way either.

My only other ideas were

• to ask user to provide a single WithPolicies<...> object itself, which might be easier to split out via SFINAE than a variadic list
• create an AllowRawPointers<T> type that is a zero-cost wrapper around T* (a struct with a single field T* which is ABI-compatible with T* itself in Wasm), so user can write things like call<AllowRawPointers<Foo>>(...) or call<void>(AllowRawPointers(ptr), 42, ...)