imp: use global paths in generated code by ibc-derive

[mina86]

Because the generated code uses paths which start with ibc::, they
are first resolved from the local module’s namespace. That is, if an
ibc module is defined, the derives create code which does not
compile. This is demonstrated by the following fragment:

use ::ibc::core::client::context::consensus_state::ConsensusState;
use ::ibc::core::commitment_types::commitment::CommitmentRoot;

mod ibc {}

#[derive(ConsensusState)]
enum AnyConsensusState { Foo(Foo) }

struct Foo;

impl ConsensusState for Foo {
    fn root(&self) -> &CommitmentRoot { todo!() }
    fn timestamp(&self) -> ::ibc::primitives::Timestamp { todo!() }
    fn encode_vec(self) -> Vec<u8> { todo!() }
}

The compilation fails with ‘failed to resolve: could not find core
in ibc’ errors.

To solve this, use global paths (i.e. ones starting with
::ibc::). Starting from 2018 Rust edition such paths resolve from
the extern prelude and thus ::ibc points at the ibc crate.

Note however that in 2015 edition global paths are anchored at the
crate root so ::ibc will attempt to look for crate::ibc
module. This means that this change is breaking for code using ancient
Rust editions.

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[Farhad-Shabani]

I don’t think we have any users on edition 2015. Let’s include it. Thanks!