glib: Enable various smallvec features

This all require a newer rustc than what smallvec depends on, but much
older than glib's minimum supported Rust version.

Enabling these features enables smallvec to use less space, work with
any array size and provide some more const functions.

Author: sdroege

glib/Cargo.toml

@@ -31,7 +31,7 @@ ffi = { package = "glib-sys", path = "sys" }
 gobject_ffi = { package = "gobject-sys", path = "gobject-sys" }
 glib-macros = { path = "../glib-macros" }
 rs-log = { package = "log", version = "0.4", optional = true }
-smallvec = "1.0"
+smallvec = { version = "1.0", features = ["union", "const_generics", "const_new"] }
 thiserror = "1"
 gio_ffi = { package = "gio-sys", path = "../gio/sys", optional = true }
 memchr = "2.5.0"