Fix map in forwards mode

[oxinabox]

This is sliced out of #221
since that stuff is going to take a bit more thinking.

but this is an obvious and easy bug fix.
Both tests errored before these changes and now they do not.

Arguably we should delete Diffractors sepecial handling of map all together and leave that to ChainRules.
But that is a bigger PR.
Also requires fixing #222 (which might be easy)

[oscardssmith]

this looks good, but you should probably define equality and hashing such that different representations of the same bundle end up equal

[oxinabox]

the PR to do that ended up being more involved than you would think.
So I will make it as a follow up

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b3e4ee0) 55.05% compared to head (fb1dd92) 55.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   55.05%   55.76%   +0.71%     
==========================================
  Files          28       28              
  Lines        2810     2819       +9     
==========================================
+ Hits         1547     1572      +25     
+ Misses       1263     1247      -16     

Files	Coverage Δ
src/tangent.jl	60.29% <90.90%> (+13.83%)	⬆️

... and 1 file with indirect coverage changes

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