Tests issued asset state tracking and contextual validation

[arya2]

This PR includes all of the changes from #26 and adds a basic test for the check::issuance::valid_burns_and_issuance function.

Unit tests for the asset_state module will be added in #26.

[dmidem]

verify_issuance_blocks_test test fails:

running 1 test
test router::tests::verify_issuance_blocks_test ... FAILED

failures:

---- router::tests::verify_issuance_blocks_test stdout ----

The application panicked (crashed).
Message:  called `Result::unwrap()` on an `Err` value: 
   0: received an error when a response was expected

Location:
   /home/dima/devel/qedit/zsa/zebra/zebra-test/src/transcript.rs:144

Error:
   0: ErrorChecker Error: block could not be full-verified due to: block has mismatched merkle root

Expected Response:
   block::Hash("055235d428e66478251b8333ba7c121a49de5960924913caa79a1a8df9e6e939")

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
Location: zebra-consensus/src/router/tests.rs:298

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.


failures:
    router::tests::verify_issuance_blocks_test

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 118 filtered out; finished in 0.38s

[dmidem]

Closed in favor of #29, as these changes have been copied there.