Better support for mainframes and EBCDIC users in general #15715

As can be seen in the photograph of a typical mainframe keyboard:

These users do not have access to curly braces. To welcome contributors also from the realm of mainframe super computing, bitcoin should use some of the lesser known features of C++: Digraphs.

This pull request replaces non-comment and non-string curly braces with their equivalent digraph representations <% and %>. I had to adjust the check-rpc-mappings.py linter a bit as it was not robust enough to deal with the full spectrum of expressivity that C++ offers.

This change should help users using EBCDIC or rather esoteric codepages like DIN_66003.

Unfortunately Trigraphs were removed from C++17, which is why I restrained from replacing symbols in string literals and comments.

Concept ACK. Though reviewing a +31933-31933 diff is a bit daunting, so I'd prefer a scripted-diff.

Aww shoot, one of the travis build spuriously errored out: https://travis-ci.org/bitcoin/bitcoin/builds/514144261

I guess the validity of the change can be checked by building without this patch and with, the binary should not change a bit.

:joy:

<!--cf906140f33d8803c4a75a2196329ecb-->Needs rebase

Happy April Fool's Day!