Use FromStr trait for parsing

[sgeisler]

Using FromStr potentially saves one clone() since the bech32 input string doesn't have to be moved into the from_string function. Since the string is never consumed inside the function this should not change the it's behavior.

[clarkmoody]

This looks great, thanks! More idiomatic Rust is always appreciated.

I'll need to see how this affects the sister rust-bech32-bitcoin library before pushing an update to Crates.io.

I'm also working on adding convert_bits to this library, according to #3

[sgeisler]

I'll need to see how this affects the sister rust-bech32-bitcoin library before pushing an update to Crates.io.

Maybe we should have a look at other API-breaking changes that might be desirable first. I'll open an issue to collect API cleanup ideas.

I'm also working on adding convert_bits to this library, according to #3

I'm looking forward to it :) I could use it to get rid of a horribly inefficient implementation of base32_to_base256 (I'm implementing BOLT 11 payment parsing)