Ability to replace crypto import

[alexwilson]

Hey,

My use-case is that I'm using react-native which doesn't currently ship with an implementation for the crypto runtime. I was wondering if it'd be possible, or if contributions would be welcome to, allow at runtime the import of crypto to be replaced with a shim?

Currently react-native-jwt exists which flat-out replaces the import with a shim, however this is not ideal.

Cheers!
Alex