Add %deprecate pragma

[mattpolzin]

Add the %deprecate pragma as proposed here.

Closes #2082.

In short, add %deprecate before a definition to cause the compiler to generate a warning whenever the given function is applied. Modify or add code documentation (||| docs) for the definition to give any additional suggestions you want printed along with the warning.

• Adds %deprecate pragma and associated warning.
• Updates CHANGELOG
• Adds beginnings of a "pragmas" page in the reference documentation.

[gallais]

Does the fact checkDeprecation is called from CompileExpr mean that we get away with
using deprecated functions in values that have a 0 quantity and are not compiled?
Should this be checked during variable lookup in TTImp.Elab.App instead?

[mattpolzin]

Does the fact checkDeprecation is called from CompileExpr mean that we get away with

using deprecated functions in values that have a 0 quantity and are not compiled?

Not sure, I'll try that and see.

Should this be checked during variable lookup in TTImp.Elab.App instead?

Quite possibly. Thanks for the idea. Where to check this was roughly a guess on my part so I only know that it works for the cases I tested but not that it's the best place to put it.

[mattpolzin]

Much better results; in broad strokes moving to Elab.App was the prefect plan, but let me know if the way I surfaced it there seems reasonable to you!