Add /blocks API

[LLFourn]

Note that all the fields have been added yet. Just some useful ones that seemed important for bdk.

[LLFourn]

thanks @tnull I addressed all comments I believe.

[tnull]

LGTM, feel free to squash.

[LLFourn]

I squashed the commits. Sorry about the epic wait on this one.

[LLFourn]

...the CI seems to be stuck.

[vladimirfomene]

Hi @LLFourn, can you rebase the changes on master and force push? That will re-run the CI jobs and I'm not able to re-run them from my end.

[notmandatory]

This will need to be rebased once #31 goes in, when testing locally running tests one at a time fixed the test reliability issue for me.

[LLFourn]

sorry everyone I was pushing to the wrong branch. Should be good to go.

[coveralls]

Pull Request Test Coverage Report for Build 3708387343

• 46 of 47 (97.87%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 81.013%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/async.rs	12	13	92.31%
<!--	Total:	46	47	97.87%	-->

Totals
Change from base Build 3702021004:	0.9%
Covered Lines:	832
Relevant Lines:	1027

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💛 - Coveralls