Use alloc instead of std

[benma]

For example in

rust-bip39/src/lib.rs

Lines 603 to 605 in a7649ae

	/// Convert the mnemonic back to the entropy used to generate it.
	#[cfg(feature = "std")]
	pub fn to_entropy(&self) -> Vec<u8> {

It would be helpful to be able to access this function (and others that return a Vec) with no_std, if one has access to alloc (alloc::vec::Vec)

A new feature alloc could be added and used instead of std for Vec.