An alternative approach to `AnchorFromBlockPosition`.

[evanlinjin]

This was done previously in #1041 but I missed it. Why can't we just have BlockPosition: Into<A> bound instead of adding our own conversion trait?

Originally posted by @LLFourn in #1172 (comment)

@evanlinjin's response:

It's because there are 3 inputs and doing A: From<(&Block, BlockId, usize)> is confusing and we won't know what the usize is for.

An alternative would be to have:

/// Parameters for constructing an [`Anchor`].
pub struct AnchorParams<'b> {
    pub block: &'b Block,
    pub block_id: BlockId,
    pub tx_pos: usize,
}

fn some_method<A: Anchor + From<AnchorParams>>() { todo!() }

What do you think? (AnchorParams will be nicely documented of course!)

[LLFourn]

I think the "alternative" is what I was suggesting. AnchorParams looks like a worse named version of BlockPosition which is what I suggested originally.