Add functional callbacks to ticker

[hreintke]

This PR adds functional callbacks to the Ticker class.
It is fully backward compatible.

I only did limited testing.
I you agree to the way of implementing I will test all options to finalize.

[codecov-io]

Codecov Report

Merging #2738 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #2738      +/-   ##
=========================================
- Coverage    27.8%   27.6%   -0.21%     
=========================================
  Files          20      20              
  Lines        3625    3655      +30     
  Branches      656     678      +22     
=========================================
+ Hits         1008    1009       +1     
- Misses       2441    2468      +27     
- Partials      176     178       +2

Impacted Files	Coverage Δ
cores/esp8266/spiffs/spiffs_nucleus.c	41.78% <0%> (-0.31%)	⬇️
cores/esp8266/spiffs/spiffs_hydrogen.c	38.06% <0%> (-0.28%)	⬇️
cores/esp8266/spiffs/spiffs_gc.c	2.57% <0%> (-0.05%)	⬇️
cores/esp8266/WString.cpp	15.13% <0%> (-0.04%)	⬇️
cores/esp8266/Print.cpp	5.03% <0%> (-0.04%)	⬇️
cores/esp8266/spiffs_api.h	44.37% <0%> (ø)	⬆️
cores/esp8266/core_esp8266_noniso.c	0% <0%> (ø)	⬆️
cores/esp8266/spiffs/spiffs_check.c	0% <0%> (ø)	⬆️
cores/esp8266/Stream.cpp	17.42% <0%> (+0.23%)	⬆️

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[devyte]

@hreintke this PR looks sort of messed up. Could you please update it, or close it and make a new one?

[hreintke]

OK, will do.

I'll keep this one open until I PR the new version.
Do you want this functional callback as single feature or should I also include the "scheduled_function" in one go.

[igrr]

I think that callback_t can be implicitly converted into std:: function<void(void)>. Maybe you could also remove the existing callback_t overload?

[hreintke]

OK will be included

[devyte]

@hreintke is this PR still valid, or is it superseded by #4209 ?

[hreintke]

@devyte This PR is indeed superseded by #4209.
I had (have) trouble getting rebase to work, that is why I made a new PR.

[devyte]

Thanks!