fix: make the no-std version work

[cryptowen]

The original version will not work since the dependencies use std by default.

[prestwich]

Updates cause cargo test --no-default-features to fail due to lack of a global allocator

[cryptowen]

[lib]
crate-type = ["cdylib", "rlib"]

What is the code above used for?
cargo test --no-default-features works without that.
And I checked some other no-std crates, they do not have that.

• https://github.com/KokaKiwi/rust-hex/blob/master/Cargo.toml
• https://github.com/rust-num/num-bigint/blob/master/Cargo.toml

[cryptowen]

And even cargo build --no-default-features fails, it works to use it as a no-std dependency.

This is an example: https://github.com/huwenchao/bitcoin-spv-lib/blob/master/contracts/bitcoin-spv-lib/Cargo.toml#L10

[prestwich]

the lib specifies linkages. See the docs here: https://doc.rust-lang.org/reference/linkage.html

[prestwich]

Check out #190 and let me know if it works for you

[cryptowen]

Check out #190 and let me know if it works for you

It works! Thanks a lot.