Temporarily export C API as Rust library as well

This is a stopgap to enable TDN to link against multiple Rust crates
without duplicating the standard library.

In the future, we'll want to separate the crate that defines the C
bindings themselves from the one defining the global allocator and panic
hook, but this works for now.

Author: nmraz

c-api/Cargo.toml

@@ -6,7 +6,7 @@ rust-version.workspace = true
 
 [lib]
 name = "spidir"
-crate-type = ["staticlib"]
+crate-type = ["staticlib", "lib"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html