Using `exfalso` and `Nat.add` for Coq 8.19

[takeschutte]

Hi,

I was having issues running this in Coq 8.19.

The following minor changes fixed the issues for me:

• Replacing plus_0_r and plus_0_l with Nat.add_0_r and Nat.add_0_l in DeBruijn.v .
• Replacing elimtype False with exfalso in Environments.v

I have made a corresponding pull request: #15