type crypto/elliptic/internal/fiat.p521Int1

one use

	crypto/elliptic/internal/fiat (current package)
		p521_fiat64.go#L44: type p521Int1 int64   // We use uint64 instead of a more narrow type for performance reasons; see https://github.com/mit-plv/fiat-crypto/pull/1006#issuecomment-892625927