all: add support for Go modules

[sbinet]

Fixes #181
Fixes #170

[cosmos72]

I have some observations:

1. requiring at least Go 1.12 seems quite strict to me. Is it intentional? I ask because the only technical restriction I am aware of is that gomacro needs Go >= 1.9
2. we currently ship a vendored version of gomacro and other dependencies. I'd rather keep the vendoring approach for a while, at least until all Go toolchains we declare to support have go modules enabled by default (in go 1.12 the user can enable them, not the project being compiled)

[sbinet]

hi @cosmos72
I've followed the convention (albeit a Gonum & Go one) of explicitly supporting+testing the current Go version N, N-1 and N-2 (ie, 1.13.x, 1.12.x and 1.11.xas of now). I don't think this means we are dropping1.9.xor1.10.x, but just that we are gently nudging people into using the better Go versions :) but, ok, I can send another PR, reintroducing 1.9.xand1.10.x` (but keeping the new ones) if you prefer.

as for vendoring, I kept the directory after reading your comment in #170 (it made sense to me as well.)