blocks: change Block to a value struct

[Jorropo]

The goal is to stop doing two allocations for each block (now it will only allocate the []byte buffer, and pass the cid.Cid, []byte pair by decomposed registers or stack).
This way of changing does not change the syntax for trivial uses of block.Block, so in theory we will have to update only producers of block.Block, not consumers.

Depends on (include) #58
Fixes ipfs/go-block-format#45

Todo:

• Test bubbling up this PR in Kubo and sees how much break.
• Test bubbling up this PR in Lotus and sees how much break.

[codecov]

Codecov Report

Merging #59 (34290c3) into main (00e0249) will increase coverage by 34.00%.
The diff coverage is 78.57%.

❗ Current head 34290c3 differs from pull request most recent head d8aae0d. Consider uploading reports for the commit d8aae0d to get more accurate results

@@             Coverage Diff             @@
##             main      ipfs/boxo#59       +/-   ##
===========================================
+ Coverage   29.75%   63.75%   +34.00%     
===========================================
  Files         100       33       -67     
  Lines       11268     1719     -9549     
===========================================
- Hits         3353     1096     -2257     
+ Misses       7547      485     -7062     
+ Partials      368      138      -230     

Impacted Files	Coverage Δ
blocks/blocks.go	64.00% <78.57%> (ø)
routing/http/server/server.go	62.60% <0.00%> (-0.33%)	⬇️
gateway/handler.go
gateway/handler_unixfs_dir.go
...l/sessioninterestmanager/sessioninterestmanager.go
bitswap/testnet/virtual.go
examples/unixfs-file-cid/main.go
...client/internal/messagequeue/donthavetimeoutmgr.go
bitswap/bitswap.go
bitswap/network/options.go
... and 59 more

[Jorropo]

Replaced by #192