chore(mimes): improve macro definitions
chore(mimes): improve macro definitions

By moving common code out of the match arms, compile times for
optimized builds are reduced by about 25% on my box, and the resulting
binary is almost 500kb smaller.

Pull request: #339
Approved by: Ryman

BenKnigge/nickel.rs,nickel-org/nickel.rs,jolhoeft/nickel.rs,anvie/nickel.rs