The `Params` struct is currently defined in the `consensus` module which
has become a collection of orthogonal consensus-ish things. We would
like to put things in more descriptive places.
The `Params` struct defines constants that are network specific so it
makes sense to put it in the `network` module. As soft proof of this
argument note in this patch how often the `Params` type is imported
along with the `Network` type.
API break:
The type is no longer available at `bitcoin::consensus::Params` but
rather is re-exported at `bitcoin::network::Params`.
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