Introduce mbedtls_pk_restart_ctx and use it

The fact that you needed to pass a pointer to mbedtls_ecdsa_restart_ctx (or
that you needed to know the key type of the PK context) was a breach of
abstraction.

Change the API (and callers) now, and the implementation will be changed in
the next commit.
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 9ab376d..a6f6a78 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -1875,7 +1875,7 @@
     {
         return( mbedtls_pk_verify_restartable( &parent->pk,
                     child->sig_md, hash, mbedtls_md_get_size( md_info ),
-                    child->sig.p, child->sig.len, &rs_ctx->ecdsa ) );
+                    child->sig.p, child->sig.len, &rs_ctx->pk ) );
     }
 #else
     (void) rs_ctx;
@@ -2653,7 +2653,7 @@
  */
 void mbedtls_x509_crt_restart_init( mbedtls_x509_crt_restart_ctx *ctx )
 {
-    mbedtls_ecdsa_restart_init( &ctx->ecdsa );
+    mbedtls_pk_restart_init( &ctx->pk );
 
     ctx->parent = NULL;
     ctx->fallback_parent = NULL;
@@ -2675,7 +2675,7 @@
     if( ctx == NULL )
         return;
 
-    mbedtls_ecdsa_restart_free( &ctx->ecdsa );
+    mbedtls_pk_restart_free( &ctx->pk );
 
     mbedtls_x509_crt_restart_init( ctx );
 }