Use the more-asserts crate in more places.

This provides assert_le, assert_lt, and so on, which can print the
values of the operands.

crates/debug/Cargo.toml

@@ -23,6 +23,7 @@ target-lexicon = { version = "0.9.0", default-features = false }
 failure = { version = "0.1.3", default-features = false }
 hashbrown = { version = "0.6.0", optional = true }
 thiserror = "1.0.4"
+more-asserts = "0.2.1"
 
 [features]
 default = ["std"]