clippy: Revisit allowed clippy rules

[lexnv]

Revisit the allowed clippy rules introduced by: #83.

A few notes on them:

• single_match: Most of the code uses match { Variant::A => ..., _ => ... }
• result_large_err: We could reduce this to propagate the expected peer, this info is not used currently in substrate

litep2p/src/error.rs

Line 115 in 08112ca

PeerIdMismatch(PeerId, PeerId),

• type_complexity: We have a few complex types to reason about futures

[dmitry-markin]

PeerIdMismatch(PeerId, PeerId) this one is used by https://github.com/dmitry-markin/p2p-probe to detect the correct PeerId.

[dmitry-markin]

Currently ignored clippy lints for context:

litep2p/src/lib.rs

Lines 21 to 29 in 5c37774

	#![allow(clippy::single_match)]
	#![allow(clippy::result_large_err)]
	#![allow(clippy::redundant_pattern_matching)]
	#![allow(clippy::type_complexity)]
	#![allow(clippy::result_unit_err)]
	#![allow(clippy::should_implement_trait)]
	#![allow(clippy::too_many_arguments)]
	#![allow(clippy::assign_op_pattern)]
	#![allow(clippy::match_like_matches_macro)]