WIP: Remove 'regex-posix' dependency

[tom-audm]

WIP, please don't merge yet!

[tom-audm]

Reasons for removing regex-posix:

• It's an additional couple of transitive dependencies which don't add much functionality (not used pervasively throughout snap-core)
• It's a dependency that's pretty orthogonal to snap-core's purpose
• It currently has no active maintainer, doesn't work with GHC 8.8, etc. https://mail.haskell.org/pipermail/libraries/2019-September/029927.html
• Its only current source form (besides Hackage) is a circa-2012 Sourceforge page

One solution is to, in addition to the work here, move assertBodyContains to the snap package, which is more batteries-included.

[tom-audm]

As described in the docs, this change requires a user who truly does want to use regexes to write:

import Text.Regex.Posix
assertBodyMatches "^Hello" (=~ "^Hello") r

instead of

assertBodyContains "^Hello" r

[mightybyte]

I'm broadly in favor of this kind of dependency pruning. The API breakage gives me pause. But it also doesn't seem like a super commonly used function. Pinging @gregorycollins.

[hvr]

@tom-audm fwiw, the 3rd and 4th bullet points are currently being addressed... ;-)

[gregorycollins]

It will break test code, but I'm probably okay with it? We'll need a major version bump unfortunately

[dten]

Just chiming in here because I wanted to build more conveniently on windows. Have you considered replacing the regex-posix dep with regex-tdfa which has no C deps
(though i'm all for removing it entirely)

[gregorycollins]

I think the consensus is that we're just going to get rid of this dependency (and the function) altogether

[mightybyte]

Yeah, I'm in favor of this change.