Implement immediate mode API

[kstrafe]

This is a PR to just get a discussion going about a potential immediate-mode API. This API does not require the user to be concerned with manual_poll or start_polling, and instead does everything via the send and recv calls (now called _immediate for convenience of implementation).

Regarding performance, we can configure whether to run resending, polling the underlying socket, handle idle clients as a function of the Nth recv/send. Ideally we can run all this at every call to avoid any unexpected "why doesn't my packet arrive?" questions.

[codecov]

Codecov Report

Merging #212 into master will decrease coverage by 0.25%.
The diff coverage is 81.08%.

@@            Coverage Diff            @@
##           master    #212      +/-   ##
=========================================
- Coverage   97.45%   97.2%   -0.26%     
=========================================
  Files          25      25              
  Lines        2321    2358      +37     
=========================================
+ Hits         2262    2292      +30     
- Misses         59      66       +7

Impacted Files	Coverage Δ
src/net/socket.rs	94.93% <81.08%> (-1.04%)	⬇️

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