capi: add 'rlib' crate type

I didn't realize this was a useful build output for a C library, but I
guess it is. Namely, it permits it to be built with other rlibs into one
giant single shared library.

Fixes #909

Author: BurntSushi

regex-capi/Cargo.toml

@@ -15,7 +15,7 @@ edition = "2018"
 
 [lib]
 name = "rure"
-crate-type = ["staticlib", "cdylib"]
+crate-type = ["staticlib", "cdylib", "rlib"]
 
 [dependencies]
 libc = "0.2"