fix(ffi): don't build C libraries by default

Remove unconditional building of FFI libraries: now
that nightly supports --crate-type, it can be opted into.
This fixes cargo's pipelining both for dependencies and
for dependent crates, resulting in faster from-scratch builds.

Cargo.toml

@@ -19,9 +19,6 @@ include = [
   #"build.rs",
 ]
 
-[lib]
-crate-type = ["lib", "staticlib", "cdylib"]
-
 [dependencies]
 bytes = "1"
 futures-core = { version = "0.3", default-features = false }