Replace J2V8 with native implementation

[friedger]

This PR

• removes the dependency of j2v8 from blockstack-sdk module

• replaces the j2v8 implenentations of the BlockstackSession API with native kotlin implementations (based on the komputing project. Some method signatures have been changed to return a value instead of taking a callback.

• separates methods calls into Blockstack (methods without a user), BlockstackSession (methods with user signed-in), BlockstackSignIn (methods for sign-in).

• fixes #154 , #159

• tests have been only changed to reflect the new method signatures.

• tests to verify the new implementation with the j2v8 implementation have been included in module blockstack-sdktest

• example apps have been updated. It shows the effort for developers:

  • update constructors
  • use BlockstackSignIn
  • wrap method calls in coroutines

[friedger]

@yknl Only a few method signatures have changed. Should we change all method signatures and remove the callback functions? Should we maintain backwards compatibility?

[friedger]

yes, some out of memory...