Review and address clippy exceptions.

[eddyb]

In order to get CI to pass I originally added these clippy exceptions:

spirt/src/lib.rs

Lines 90 to 117 in b586de4

	// crate-specific exceptions:
	#![allow(
	// FIXME(eddyb) review all of these (some of them are intentional, sadly).
	clippy::collapsible_if,
	clippy::comparison_to_empty,
	clippy::get_first,
	clippy::iter_nth_zero,
	clippy::map_err_ignore,
	clippy::match_same_arms,
	clippy::match_wildcard_for_single_variants,
	clippy::needless_borrow,
	clippy::needless_lifetimes,
	clippy::nonminimal_bool,
	clippy::redundant_closure,
	clippy::redundant_closure_call,
	clippy::redundant_field_names,
	clippy::semicolon_if_nothing_returned,
	clippy::should_implement_trait,
	clippy::single_match,
	clippy::single_match_else,
	clippy::string_add,
	clippy::unimplemented,
	clippy::unnested_or_patterns,
	clippy::unused_self,
	clippy::useless_conversion,
	clippy::vtable_address_comparisons,
	elided_lifetimes_in_paths,
	)]

Most of them should be addressable, though there might strangely opinionated ones, or clippy bugs, that may require #[allow(...)]-ing some lints still.