Merge branch 'patch-1' into 'master'

Update index.d.ts; specify PRIVATE_KEY_LENGTH const export

See merge request matrix-org/olm!15

javascript/index.d.ts

@@ -125,3 +125,5 @@ declare class SAS {
 export function init(opts?: object): Promise<void>;
 
 export function get_library_version(): [number, number, number];
+
+export const PRIVATE_KEY_LENGTH: number;