The changes do not affect the public API:
`RandomBuffer` was used internally for secure randomness generation before
`crypto.getRandomValues` was made available to WebWorkers, requiring
generating randomness in the main thread.
As a result of the change, the internal `getRandomBytes()` and some functions
that use it are no longer async.
In the lightweight build, lazily load bn.js only when necessary.
Also, use Uint8Arrays instead of strings in PKCS1 padding functions, and
check that the leading zero is present when decoding EME-PKCS1 padding.
