Fix build error when alloc feature is disabled

[taiki-e]

Fixes build error by the way mentioned in #10 (comment).

print only variant name when alloc feature is disabled? (like the following)

#[cfg(feature = "alloc")]
let name = format!("{}::{}", self.name, variant.name());
#[cfg(not(feature = "alloc"))]
let name = variant.name();

[taiki-e]

Oh, our MSRV is 1.51 (because we use min const generics), so we can use resolver = "2" that the more preferable than --no-dev-deps (that edit Cargo.toml).

UPDATE: done in 013dca6