Replace openssl/sha2 with a sha256 implementation

[elichai]

This is to remove openssl as a dependency, making this tool entirely free standing
the sha256 implementation is from libsecp256k1(https://github.com/bitcoin-core/secp256k1/blob/master/src/hash_impl.h) but I've adjusted it to the code style here.

There are no tests here, but I manually sampled a few transactions and blocks and verified that their hash matches what's in blockstream.info

There doesn't seem to be any performance difference as most of the work is in I/O:

Before:

$ time ./bitcoin-iterate -q && time ./bitcoin-iterate -q
bitcoin-iterate: Skipped 4681563 at end of /home/elichai2/.bitcoin/blocks/blk02222.dat
                                                           
real    2m30.215s                     
user    0m1.315s                                         
sys     0m13.255s                                   
bitcoin-iterate: Skipped 4681563 at end of /home/elichai2/.bitcoin/blocks/blk02222.dat
                                                           
real    2m28.074s                                                                                                     
user    0m1.322s                                                                                                      
sys     0m13.238s

After:

$ time ./bitcoin-iterate -q && time ./bitcoin-iterate -q                                                                                                                               
bitcoin-iterate: Skipped 4681563 at end of /home/elichai2/.bitcoin/blocks/blk02222.dat
                                                                                                                      
real    2m30.056s                                                                                                     
user    0m1.401s                                                                                                      
sys     0m13.151s                                                                                                     
bitcoin-iterate: Skipped 4681563 at end of /home/elichai2/.bitcoin/blocks/blk02222.dat
                                                                                                                      
real    2m26.071s                                                                                                     
user    0m1.467s                                                                                                      
sys     0m13.074s                                                                                                     

This should resolve #16 and resolve #11

[renepickhardt]

this looks good to me but of course this is for @rustyrussell to decide. Did not expect that my issue would actually get a PR or any fix at all. Kind of only put it there if beginners use the code and google the error.

[elichai]

It took me 5-10 minutes of work, so why not,
And it's obviously fine if @rustyrussell decides to leave this as is (it still works great even after years)

[jsarenik]

Tested ACK 9ddef39 (processed all testnet3 blocks)