Add uint256 fuzz test vs TTMath

[mratsim]

This adds fuzz testing vs TTMath.

It requires the latest Nimbus patch: status-im/Nim@60db1a8

There is still a C++ only SIGSEGV with nil string/json (?):

<...>/.nimble/pkgs/quicktest-0.18.0/quicktest.nim(414, 47) Hint: simpleGetOrDefault(parseJson(serialized397017), "args") --> 'getOrDefault(parseJson(serialized397017), "args")' [Pattern]
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Also unfortunately, to add this to CI, Stint must require quicktest and ttmath, though if they are not used it shouldn't force C++ compilation. Feature request here: nim-lang/nimble#482.