Bump log from 0.4.14 to 0.4.15

Sourced from log's changelog.

[0.4.15] - 2022-02-23

• Silence a warning about the deprecated spin_loop_hint.
• Relax ordering in the atomic set_max_level call.
• Add thumbv4t-none-eabi to targets that don't support atomics
• Allow levels to be iterated over.
• Implement Log on some common wrapper types.
• Improvements to test coverage.
• Improvements to documentation.
• Add key-value support to the log! macros.
• Tighten kv_unstable internal dependencies so they don't bump past their current alpha.
• Add a simple visit API to kv_unstable.
• Support NonZero* integers as values in structured logging
• Support static strings as keys in structured logging

• bc181df Merge pull request #492 from rust-lang/KodrAus-patch-2
• cf20336 update changelog for 0.4.15 with last minute additions
• 59fa5dc Merge pull request #491 from Thomasdezeeuw/kv-value-nonzero
• 4f9a212 Implement ToValue and From<$ty> for Value for NonZero
• d672066 Merge pull request #488 from wtetzner/master
• 863c461 Merge pull request #486 from Thomasdezeeuw/issue#484
• cb20e45 #487 Fix formatting.
• 8038ea8 #487 Add thumbv4t-none-eabi to the list of targets that don't support atomics.
• c626f82 Support static string as key value
• 21e30fc Merge pull request #485 from KodrAus/cargo/0.4.15
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