features: drop error_in_core

Rather, predicate it on lack of std. This change supports use of `cargo
test --all-features`.

Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

Author: amboar

Cargo.toml

@@ -25,7 +25,6 @@ std = ["deku_derive/std", "bitvec?/std", "alloc", "no_std_io/std"]
 alloc = ["bitvec?/alloc"]
 logging = ["deku_derive/logging", "log"]
 no-assert-string = ["deku_derive/no-assert-string"]
-error_in_core = []
 bits = ["dep:bitvec", "deku_derive/bits"]
 
 [dependencies]

src/error.rs

@@ -113,5 +113,5 @@ impl From<DekuError> for std::io::Error {
     }
 }
 
-#[cfg(feature = "error_in_core")]
+#[cfg(not(feature = "std"))]
 impl core::error::Error for DekuError {}