Explicitly enable indexmap's "std" feature

The indexmap crate uses the autocfg crate to do target detection to
determine whether `std` is available. Arrow isn't targeting `no_std`
environments, so the target detection isn't necessary. This might save
some build time.

indexmap-rs/indexmap#145

arrow/Cargo.toml

@@ -40,7 +40,7 @@ path = "src/lib.rs"
 serde = { version = "1.0", features = ["rc"] }
 serde_derive = "1.0"
 serde_json = { version = "1.0", features = ["preserve_order"] }
-indexmap = "1.6"
+indexmap = { version = "1.6", features = ["std"] }
 rand = { version = "0.8", optional = true }
 num = "0.4"
 half = "1.8"