[aurora/src] Remove requirement on nightly experimental features

https://doc.rust-lang.org/beta/core/ffi/type.c_size_t.html

> This type is currently always usize, however in the future there may
> be platforms where this is not the case.

It seems rust-lang/rust#88345 has stalled for
over a year now.
For now let's assume this is `usize`

Author: romen

aurora/src/osslparams.rs

@@ -173,7 +173,16 @@ pub const OSSL_PARAM_END: ossl_param_st = ossl_param_st {
     return_size: 0,
 };
 
-const OSSL_PARAM_UNMODIFIED: usize = core::ffi::c_size_t::MAX;
+/*
+ * core::ffi:c_size_t is only in nightly, and unstable
+ *
+ * https://github.com/rust-lang/rust/issues/88345 seems to have stalled,
+ * so for now we just assume c_size_t and usize are the same.
+ *
+ * TODO: revisit if c_size_t goes stable
+ */
+// const OSSL_PARAM_UNMODIFIED: usize = core::ffi::c_size_t::MAX;
+const OSSL_PARAM_UNMODIFIED: usize = usize::MAX;
 
 pub fn ossl_param_locate<'a>(params: &'a mut [OSSLParam], key: &KeyType) -> Option<&'a mut OSSLParam> {
     params.iter_mut().find(|p| {