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/environ/Cargo.toml

@@ -30,6 +30,7 @@ log = { version = "0.4.8", default-features = false }
 zstd = "0.5"
 toml = "0.5.5"
 file-per-thread-logger = "0.1.1"
+more-asserts = "0.2.1"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"