Merge #16566: util: refactor upper/lowercase functions

0481fa25844dc6ec9f6c3fac8428d874d34b0ad0 util: refactor upper/lowercase functions (Karl-Johan Alm)

Pull request description:

  This includes renaming Downcase() to ToLower() and make it return a string rather than modify referenced arg.
  Also adds ToUpper() string version.

  Additionally, it clarifies that the locale independency of the case functions is a *feature* and not a limitation. I interpreted it as the latter and rewrote code to be locale-aware before realizing this.

  This is done in preparation for #11413 and as a general refactor. I don't think the optimization that the pre-refactor state gave warrants the unwieldy usage.

ACKs for top commit:
  laanwj:
    ACK 0481fa25844dc6ec9f6c3fac8428d874d34b0ad0 from me
  practicalswift:
    ACK 0481fa25844dc6ec9f6c3fac8428d874d34b0ad0 -- diff looks correct
  l2a5b1:
    ACK 0481fa2 - Although, I think @luke-jr's [feedback](https://github.com/bitcoin/bitcoin/pull/16566#issuecomment-519580760) is spot on; `Downcase` is just an artifact of `ParseNetwork`, which has been happily downcasing `net` via a string argument for over 7 years; and I do recommend to add `ToLower` *when* somebody actually needs it in new work, there is no point in keeping a trivial utility function if it is not appreciated.
  promag:
    ACK 0481fa25844dc6ec9f6c3fac8428d874d34b0ad0.

Tree-SHA512: 9b834ecc1b97db043e261bcbc59e42372e11e2fb9a6943688f18a835bf5c9205f68e4614f58e90ba260d1b8f0e060c6f67b390b62436c21b56891db23bc41628

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