Use core replacements for std members

[GeneFerneau]

In preparation for no_std build support, replace std structs and functions with core equivalents

[devrandom]

ACK

[codecov]

Codecov Report

Merging #926 (ec3739b) into main (b6de281) will decrease coverage by 0.01%.
The diff coverage is 90.62%.

@@            Coverage Diff             @@
##             main     #926      +/-   ##
==========================================
- Coverage   90.48%   90.46%   -0.02%     
==========================================
  Files          59       59              
  Lines       29896    29896              
==========================================
- Hits        27050    27046       -4     
- Misses       2846     2850       +4     

Impacted Files	Coverage Δ
lightning/src/chain/chainmonitor.rs	95.52% <ø> (ø)
lightning/src/chain/mod.rs	100.00% <ø> (ø)
lightning/src/ln/chan_utils.rs	97.34% <ø> (ø)
lightning/src/ln/channelmanager.rs	83.55% <ø> (+0.08%)	⬆️
lightning/src/ln/features.rs	98.83% <ø> (ø)
lightning/src/ln/functional_tests.rs	96.80% <ø> (-0.10%)	⬇️
lightning/src/ln/msgs.rs	88.22% <ø> (ø)
lightning/src/ln/onion_route_tests.rs	96.80% <ø> (ø)
lightning/src/ln/onion_utils.rs	94.90% <ø> (ø)
lightning/src/ln/reorg_tests.rs	99.62% <ø> (ø)
... and 23 more

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