Allow user defined block hooks when using two step write for selective cars

[hannahhoward]

Goals

This is a simple but pretty obscure feature. When building cars with Selectors, we have an option for a two step write -- one step performs the traversal, while another writes the traversed cids to a file. This is important among other things so that we can get a total file size ahead of time. We also have an option in the 1-step write to call a user defined hook as we write blocks. Now, we allow the two step write to use these user-defined hooks. However, they are not called till you actually dump the file. For a variety of reasons, we need this for Filecoin, but it seems to me to be a non-controversial enough change as to allow us to merge to master?

Implementation

• hold the user funcs in the intermediate structure
• track offset/size and call during io.Write

[codecov]

Codecov Report

Merging #37 (7f9fecd) into master (c2f1ff2) will increase coverage by 0.95%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   50.20%   51.16%   +0.95%     
==========================================
  Files           3        3              
  Lines         245      258      +13     
==========================================
+ Hits          123      132       +9     
- Misses         87       89       +2     
- Partials       35       37       +2     

Impacted Files	Coverage Δ
selectivecar.go	65.83% <71.42%> (+0.41%)	⬆️

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[rvagg]

sgtm

there's a loss of coverage here though because the Write() and Prepare() without hook case isn't being covered anymore

[hannahhoward]

I merged cause there's not a seperate code path for no-hooks -- it's just simpler