[Plat-8106] Remove calls to libc printf

[kstenerud]

Goal

Libc printf acquires locks, and so it not safe for use in async-safe code.

This PR replaces these calls with async-safe alternatives, and also disables low level logging by default so that it doesn't run in production (it doesn't hurt anything, but bloats the binary size, and is of no use in the field).

Design

Low level number-to-string functions from the JSON encoder have now been exposed so that they can be used from elsewhere.

Added stb_sprintf to replace the few sprintf calls in the logger and other places in the codebase that can't be easily replaced with simpler alternatives.

All low level logging is now dependent upon the define BSG_KSLOG_ENABLE_LOW_LEVEL_LOGGING. When 0, no low level logging code is compiled.

Testing

Added tests for the low level number-to-string functions.
Tested manually with BSG_KSLOG_ENABLE_LOW_LEVEL_LOGGING = 1 and = 0.

[github-actions]

Bugsnag.framework binary size decreased by 536 bytes from 819,056 to 818,520 🎉

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  +2.7%     +96  +2.7%     +96    __TEXT,__gcc_except_tab
  +0.3%     +24  +0.2%     +32    [__DATA]
  [ = ]       0  -0.1%      -8    __DATA,__bss
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  -2.1% -5.36Ki  -2.1% -5.36Ki    __TEXT,__text
  -0.1%    -536  [ = ]       0    TOTAL

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