Buffer capture file reads

[godlygeek]

#1011 accidentally targeted the faster-capture-padding-scan branch, rather than main.

While fixing that, I got nerd sniped into playing around, since I wasn't totally happy with how #1011 was factored, and came up with something that I think is organized better and which is actually faster as well.

This PR removes the buffering that was happening inside the SocketSource in favor of a new BufferedSource class, which wraps an existing Source and adds buffering. The RecordReader only ever reads via a BufferedSource, and the BufferedSource isn't a subclass of the Source abstract base class, so the calls made by RecordReader in its hot loop don't need to go through virtual dispatch. We only need to hop through a vtable when we need to refresh the buffer, which is once per 64KiB read for the FileSource, though more often for the SocketSource since we don't expect the socket to have 64KiB sitting in it waiting for us every time we request more data.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.48%. Comparing base (b4524ee) to head (7fb1ca7).

Files with missing lines	Patch %	Lines
src/memray/_memray/source.cpp	98.38%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1016      +/-   ##
==========================================
+ Coverage   92.42%   92.48%   +0.05%     
==========================================
  Files         102      102              
  Lines       13303    13312       +9     
  Branches      482      482              
==========================================
+ Hits        12295    12311      +16     
+ Misses       1008     1001       -7     

Flag	Coverage Δ
cpp	92.48% <98.43%> (+0.05%)	⬆️
python_and_cython	92.48% <98.43%> (+0.05%)	⬆️

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