MSRV of proc_macro2 is 1.31

[thomaseizinger]

Currently, the build against Rust 1.24 fails because proc_macro2 requires Rust 1.31.

The dependency is brought in by serde_derive:

If we want to keep supporting Rust 1.24, it seems like the options are:

• drop serde_derive (and reimplement the serialization manually)
• pin it to a version that compiles on Rust 1.24 (rust-bitcoin pins it to <1.0.99)

Neither of them seem to be like a great idea:

• dropping serde_derive means we'd have to write a lot of boilerplate
• pinning the version limits downstream users in which features of serde_derive they can access

What are your thoughts on supporting Rust 1.24 in this repo @stevenroose ?
Currently, none of the PRs (like #80) can be merged because of the failing build for Rust 1.24.

[thomaseizinger]

Ping @stevenroose are there any plans on resolving this? bitcoincore-rpc depending on an outdated version of rust-bitcoin is currently blocking us from updating rust-bitcoin (which we'd really want, esp. with the new-types coming in on the next release).