Amends esp8266#8507
I took the liberty to also do some refactoring; specifically, fixing signed vs. unsigned mismatch in len, using pointer object vs. the original manual malloc & free, try to have named constants for certain addresses and lengths, plus localize printing of u8 arrays.

The suggested test to have a 'dummy' verifier works just fine. (...how it actually works and gets the hash to compare with is a whole other question, though)

Another issue noticed while testing, in the underlying bearssl api there's an actual limit for hash length.
https://github.com/earlephilhower/bearssl-esp8266/blob/6105635531027f5b298aa656d44be2289b2d434f/inc/bearssl_rsa.h#L257