Match the behavior of peek_slice to recv_slice

[hunhoffe]

Previously, peek_slice() did not match the behavior of recv_slice() when there is a buffer wrapparound. I believe that by using the read_allocated() method, this will be fixed. I created an issue for this here: #833

This is my first time trying to contribute to smoltcp so I've only tested locally a few ways. One is running my program that uses peek_slice() that previously failed (it now succeeds). I also ran:

cargo +stable test --no-default-features --features alloc,socket-tcp,proto-ipv4,medium-ethernet

And the tests completed succesfully. Please let me know if I need to do anything else.

[Dirbaio]

Looks good! Can you add a test asserting the new behavior? ie test that it does return data past the wraparound.

[codecov]

Codecov Report

Merging #834 (953ee32) into main (01e1acb) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #834      +/-   ##
==========================================
+ Coverage   79.52%   79.58%   +0.05%     
==========================================
  Files          78       78              
  Lines       27762    27809      +47     
==========================================
+ Hits        22078    22131      +53     
+ Misses       5684     5678       -6     

Files Changed	Coverage Δ
src/socket/tcp.rs	96.62% <100.00%> (+0.13%)	⬆️

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[Dirbaio]

thanks!