Reduce compilation times

[fthomas]

This tweet https://twitter.com/missingfaktor/status/909064863917510656 shows that increased compilation time with refined is a major issue. We should investigate ways to get better in this regard.

[fthomas]

The first step towards this goal has been merged: ~26% reduced compilation times with #332.

[fthomas]

Further steps in this direction: #333 and #334

[fthomas]

General performance improvements: #335, #336, #337, and #338

[fthomas]

Reduced compilation time when refining with full refined types: #388