Update to serde 1.0

[jeandudey]

Status: Ready for review

---

[apoelstra]

I think upgrading to serde 1.0 might be easier than trying to support both. I believe what needs to be done is:

1. Update strason to 0.4 and secp256k1 to 0.10.0.
2. Put strason, serde and the whole decimal module under a feature gate. (Later we can change decimal to be compatible with serde_jsons arbitrary_precision hack, drop the strason dependency, and make serde_json be a dev-dependency. But one step at a time.)
3. Replace the serde_struct_impl macro in src/internal_macros.rs. You'll find the hardest thing to replicate is that it tries to recover from a missing field by calling MapVisitor::missing_field. The modern way[*] to do this is to require Default be implemented on optional fields, and use Default::default() in place of missing_field. But as far as I can tell we never have optional fields in rust-bitcoin structs, so it's fine to just drop this "feature".

[*] Ok, yes, the actual modern way is to use serde_derive, but this macro lets us avoid the compiler version bump.

[jeandudey]

@apoelstra strason 0.4 isn't compiling on 1.14.0

[jeandudey]

Also I've included your secp256k1 changes in this PR.

[apoelstra]

Can you rebase now that we have secp 0.10 in?

[jeandudey]

Done

[apoelstra]

I think this any isn't needed because serde-decimal implies strason?

[apoelstra]

ACK except for this. I think it can just be #[cfg(feature = "strason")]

[jeandudey]

Done