rust version 1.51.0 can't compile haybale

[YuXinFan]

It's just a tip for new users:
After updating rust to 1.51.0, haybale version 0.6.3 can't be compiled.
But haybale works well at rust 1.50.0
It seems caused by rust's backward incompatibility.

[cdisselkoen]

A couple days ago I filed an issue with one of haybale's dependencies (dtolnay/reduce#4) hoping they would resolve this. Since they haven't responded yet, I may just put in a workaround on haybale's side so that it can compile with both 1.50 and 1.51.

[cdisselkoen]

Fixed and released haybale version 0.6.4.