fix tinythread memory leak

[kevinushey]

Closes #185. See also: #186

@dipterix, can you take a look and let me know what you think?

[dipterix]

I think this PR is pretty legit, much better than mine.

Also just FYI: Merged into ravetools version dipterix/ravetools@801068a

Tested on the following platform

• MacOS (Intel & M2)
• Linux (Docker, rocker)

The memory usage falls back once gc() is called.

In addition UBSAN-Clang check results:

# R -d "valgrind --tool=memcheck --leak-check=full" -f tmp.R
==13== Memcheck, a memory error detector
==13== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==13== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright info
==13== Command: /usr/lib/R/bin/exec/R -f tmp.R
==13== 
> library(ravetools)
> 
> ravetools::fast_mvmedian(matrix(1:100,10))
 [1]  5.5 15.5 25.5 35.5 45.5 55.5 65.5 75.5 85.5 95.5
> 
> # testthat::test_dir('tests')
> # testthat::test_examples('.')
> 
> 
==13== 
==13== HEAP SUMMARY:
==13==     in use at exit: 52,590,480 bytes in 10,325 blocks
==13==   total heap usage: 29,524 allocs, 19,199 frees, 89,546,893 bytes allocated
==13== 
==13== LEAK SUMMARY:
==13==    definitely lost: 0 bytes in 0 blocks
==13==    indirectly lost: 0 bytes in 0 blocks
==13==      possibly lost: 0 bytes in 0 blocks
==13==    still reachable: 52,590,480 bytes in 10,325 blocks
==13==                       of which reachable via heuristic:
==13==                         newarray           : 4,264 bytes in 1 blocks
==13==         suppressed: 0 bytes in 0 blocks
==13== Reachable blocks (those to which a pointer was found) are not shown.
==13== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==13== 
==13== For lists of detected and suppressed errors, rerun with: -s
==13== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)