Restrict proc-macro2 to Version 1.0.86

[AdinAck]

I noticed that proc-macro2 version 1.0.87 introduced character validation for the Punct struct, resulting in svd2rust panicking like so:

thread 'main' panicked at {me}/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.87/src/lib.rs:847:13:
unsupported proc macro punctuation character '{'

Temporary solution is holding back proc-macro2 at version 1.0.86. Better solution would probably be representing the { character with a different struct than Punct.

[burrbull]

Fixed in #864