Deduplicate warnings

[miraclx]

Fixes #67

Hides warnings at the ABI generation stage only when building contracts

Before	After

[miraclx]

@itegulov, when you get the time please can you test this. My one test in the screenshot worked, but ever since, I've been unable to get any warnings at all through cargo near, still investigating why. Thought it was a caching thing, but I've cleaned by target folder repeatedly and still nothing. And yes, normal cargo build reports errors.

_{maybe it's the meds I'm on}

[itegulov]

@miraclx yeah I am not getting any warnings either

[miraclx]

Fixed in 5378499 (#65), merged in here.

[itegulov]

Can confirm that warnings appear now!

[austinabell]

curious, why use a map instead of just a vec of (K, V) if you're not checking the duplication of keys on insert?

[miraclx]

Well, actually, for RUSTFLAGS, we're checking to ensure we don't replace previous entries, but instead concatenate them. So it's here to allow us quickly index the map, instead of having to iterate the whole Vec<(K, V)>.