Add custom profile to set lto&codegen units *only* with building the contracts?

[matklad]

Today we specify lto=true in Cargo.toml. This is pretty important to get fast&lean mainnet.wasm, but it also means that, when we build the tests with release, we compile nearcore (actually, two nearcores) with lto, which takes ages.

Modern rust has custom profiles, so we can add a contract profile for compiling aurora to wasm, and stick to the default release for tests & standalone runner.