Remove sliceiter

[andy-thomason]

This PR removes SliceIter and replaces Real and Int with Doubles and Integers.

[Ilia-Kosenkov]

I would expect to have here <Item = &Rfloat>, similar to as_logical_iter

[andy-thomason]

True. Another PR, though.

[Ilia-Kosenkov]

I had conflicts with these

extendr/extendr-api/src/robj/into_robj.rs

Lines 300 to 324 in 4367764

	impl ToVectorValue for Bool {
	fn sexptype() -> SEXPTYPE {
	LGLSXP
	}
	fn to_logical(&self) -> i32
	where
	Self: Sized,
	{
	self.0
	}
	}
	impl ToVectorValue for &Bool {
	fn sexptype() -> SEXPTYPE {
	LGLSXP
	}
	fn to_logical(&self) -> i32
	where
	Self: Sized,
	{
	self.0
	}
	}

And then I think here

extendr/extendr-api/src/robj/into_robj.rs

Lines 341 to 345 in 4367764

	fn fixed_size_collect<I>(iter: I, len: usize) -> Robj
	where
	I: Iterator,
	I: Sized,
	I::Item: ToVectorValue,

Will ToVectorValue remain in the codebase or is it subject to retirement?

[andy-thomason]

I'm inclined to retire ToVectorValue in due course. It is doing a lot of work at the moment.

There are changes coming to the language that will allow better specialisation of traits.
Don't hold your breath, though.