Informative assert messages

[wilzbach]

i.e. rebasing dlang/dmd#8517

[marler8997]

Can be merged anytime

• Makes the fork more appealing

[wilzbach]

This has been partially merged at mainline DMD now (druntime changes are still pending -dlang/druntime#2413), though it's still behind a switch and will be so for probably a long time.

So I guess -checkaction=context is another of those flags which should be set by default.

[wilzbach]

Closing in favor of #76 as it's now just about flipping the default dmd.globals (or adding the flags in a wrapper).