Update build & test infrastructure

[petdance]

Switch to Makefile.PL instead of Build.PL

Add Test::Warnings to all the tests to check for regressions. (Test::Warnings is what I use at work, but if there's a reason to use Test::NoWarnings instead, I'm all ears.)

[oalders]

I asked on #toolchain on irc.perl.org and the feedback I got was that Test::Warnings > Test::NoWarnings and that Test2 also has a nice way to handle this: https://metacpan.org/pod/Test2::Tools::Warnings

[petdance]

I added Test::Warnings. Still using Build.PL for now.