Change names of address variables from "a" to "addr"

[jadephilipoom]

As per discussion in #36

In several Coq lemmas I use a as the name for function arguments that have address types. I should go through, find these places, and change them to either the same argument name as the source C code or to something clearer like addr or ipaddr when there isn't an analogous variable in the source.

[hkim15]

Not urgent - something that I just noticed while reading your code. I usually avoid single-character variables to improve readability.