Dummy sync implementation for no_std

[devrandom]

We would like to not bother synchronizing access on no_std platforms, because there is no mature rust multi-threading support for them yet. Accordingly, this PR implements drop-in replacements for Mutex, Condvar and (not done yet) RwLock.

See also discussion in #842 (comment) - but this PR uses a slightly different approach.

[codecov]

Codecov Report

Merging #1008 (2e8f4fe) into main (1f1d7c6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
- Coverage   90.76%   90.76%   -0.01%     
==========================================
  Files          60       60              
  Lines       30909    30909              
==========================================
- Hits        28056    28055       -1     
- Misses       2853     2854       +1     

Impacted Files	Coverage Δ
lightning/src/chain/chainmonitor.rs	95.80% <ø> (ø)
lightning/src/chain/channelmonitor.rs	90.80% <ø> (ø)
lightning/src/ln/chanmon_update_fail_tests.rs	97.94% <ø> (ø)
lightning/src/ln/channelmanager.rs	84.85% <ø> (ø)
lightning/src/ln/functional_test_utils.rs	94.87% <ø> (ø)
lightning/src/ln/functional_tests.rs	97.34% <ø> (-0.02%)	⬇️
lightning/src/ln/peer_handler.rs	46.44% <ø> (ø)
lightning/src/routing/network_graph.rs	91.81% <ø> (ø)
lightning/src/util/enforcing_trait_impls.rs	90.38% <ø> (ø)
lightning/src/util/logger.rs	82.14% <ø> (ø)
... and 7 more

---

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[TheBlueMatt]

Cool! Glad it kinda just worked. CI should also at least run cargo test --no-default-features --features no_std (it currently just runs with hashbrown explicitly), but that seems to pass locally.

[devrandom]

Cool! Glad it kinda just worked. CI should also at least run cargo test --no-default-features --features no_std (it currently just runs with hashbrown explicitly), but that seems to pass locally.

I read what you said as "instead of --features hashbrown", but let me know if I misunderstood. Will do.

[jkczyz]

ACK 2e8f4fe