Add demangle support using external rustfilt tool

[chrysn]

This PR adds support for an external demangler: If rustfilt is present, symbol names are shown in a demangled way. This should cover not only Rust but also C++ symbols.

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This is moving haukepetersen#12 in here; marked as a draft as I'll have yet to review whether this is still all current.

[chrysn]

Rebased and re-tested: Symbols that previously looked like _ZN123_$LT$coap_handler_implementations..wkc_implementation..WellKnownCore$LT$H$GT$$u20$as$u20$coap_handler..handler..Handler$GT$14build_response17hdc5d4b38dfaaba18E now look like <coap_handler_implementations::wkc_implementation::WellKnownCore<H> as coap_handler::handler::Handler>::build_response (which is just as long but at least readable).

This does emit a warning about not having been able to run the demangler even when there are no Rust symbols, but then I don't know how to reliably recognize them, it's just a warning, and it's not like there's certainty that there are no surprise Rust modules in RIOT any more ;-)

Please review.

[mguetschow]

Tested locally (or rather with BUILD_IN_DOCKER=1) with examples/rust-gcoap and can confirm this to work nicely.

All rust symbols are grouped under "unspecified", but that was already the case before so out of scope for this PR, I'd say.

Just a minor suggestion below.

[mguetschow]

Thanks!

[mguetschow]

Oh wait, the commit message should be changed with s/rustfix/rustfilt/

[chrysn]

Thanks for checking, fixed.

Applied the commit message suggestion