compilation fix for no-std

[michalkucharczyk]

This PR fixes following compilation problem for no-std environments:

error[E0412]: cannot find type `Vec` in this scope
   --> /home/miszka/parity/10-genesis-config/rust-bip39-release/src/lib.rs:110:26
    |
110 |     pub fn to_vec(&self) -> Vec<Language> {
    |                             ^^^ not found in this scope

error[E0599]: no method named `to_string` found for struct `Decompositions` in the current scope
    --> /home/miszka/parity/10-genesis-config/rust-bip39-release/src/lib.rs:201:42
     |
201  |             *cow = Cow::Owned(cow.as_ref().nfkd().to_string());
     |                                                   ^^^^^^^^^ method not found in `Decompositions<Chars<'_>>`

The problem can be reproduced with the following steps:

cargo new rust-bip39-dep-problem2
pushd rust-bip39-dep-problem2
cargo add bip39 --git "https://github.com/rust-bitcoin/rust-bip39"  --rev b100bf3 --no-default-features --features alloc
cargo build

[michalkucharczyk]

cc: @tcharding

Would be nice to add this check to CI somehow. Do you have any idea how to do it?
Maybe we could create a cargo new somewhere in tmp directory?

[tcharding]

ACK 8653fb2

[zhang-wenchao]

Can you merge this？

[tcharding]

Would be nice to add this check to CI somehow. Do you have any idea how to do it?

Saw this comment because PR turned up in my notifications. Dunno if this is just as stale comment but there is a check for this in CI ATM on master.

[tcharding]

Can you merge this？

cc @stevenroose