No_std support for floresta-common

[JoseSK999]

I have tried to make floresta-common be able to compile without the standard library.

1. Added the missing no-std feature in Cargo.toml, together with a default std feature.
2. Added the missing core2 and hashbrown dependencies (used in the no-std prelude).
3. sha2, miniscript and bitcoin default features disabled to avoid importing std-lib, although our new std feature enables it.
4. Also added a new descriptors-no-std feature to use them without the std-lib.

It should be able to compile without std-lib by running cargo build --no-default-features --features no-std, but we get an error with the current fork of the bitcoin crate. Concretely at bitcoin/src/consensus/encode.rs:

impl_vec!(UtreexoBlock);
impl_vec!(CompactLeafData);

UtreexoBlock and CompactLeafData are not found in the scope. The first one is because the import requires the std-lib, the second one perhaps because of the impl_consensus_encoding! macro used for CompactLeafData.

The rest of floresta crates use the std version of floresta-common, just as before, so this change shouldn't break current things.

[Davidson-Souza]

UtreexoBlock and CompactLeafData are not found in the scope. The first one is because the import requires the std-lib, the second one perhaps because of the impl_consensus_encoding! macro used for CompactLeafData.

Wait, I should've removed this. We have this implemented in floresta-wire. What do you think about removing it from our rust-bitcoin fork?

[JoseSK999]

In floresta-chain no? If we have the types and implementations duplicated sure, it is a good idea! Why else do we need the bitcoin fork?

[jaoleal]

Dealing with messages, somethings we would have to reimplement...

That actually might be easy to do since davidson already did it in the fork.

[Davidson-Souza]

@JoseSK999 #258 removes all this unused code. Does this PR compile no-std after that?

[JoseSK999]

Yes @Davidson-Souza, if I apply this commit on top of yours it compiles with no-std.

[Davidson-Souza]

Yes @Davidson-Souza, if I apply this commit on top of yours it compiles with no-std.

Nice! I'll merge my pr and you rebase on it

[JoseSK999]

Done!