Add assertions

GHC currently has a nice feature for assertions, which allow the programmer to insert a condition which is asserted to be true. If the condition is in fact false, an error is raised which contains the source code line of the assertion. A compiler flag allows all assertions to be disabled.

http://www.haskell.org/ghc/docs/latest/html/users_guide/sec-assertions.html

Possible variations:

1. Have a form of 'assert' which can take an arbitrary string to which the source file location is prepended.
2. assert can tie in to imprecise exceptions, if they are adopted, or a failed assertion can be bottom
3. Remove the source line feature (makes the proposal simpler, but less useful because now all you get is the compiler flag to turn assertions off)

Trac metadata

Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	HaskellPrime
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Trac import + 1 deleted label

assigned to @trac-none and unassigned @trac-ijones

Trac metadata

Trac field	Value
Component	HaskellPrime → Proposal

changed milestone to %⊥

moving non-milestoned many year old legacy tickets out of the way

added enhancement modification labels and removed modification label

Set default priority (as this confuses Trac otherwise)