* Move Context into its own file
Just the core definition - this is so that we have access to it in
Core.Core, for inclusion in error messages, to save normalisation of
terms in errors until we actually show them.
* Normalise errors on display, not when they arise
This can save a lot of time in ambiguity resolution if the errors are
complicated, because the errors might never be displayed if it's in an
abandoned branch.
This involves lifting 'Context' out of Core.Context, because we need to
store it in Error, which is needed by Core, which in turn is needed in
Core.Context.
Also moved a couple of test caes from ttimp to idris2, so that the
errors get rendered properly and won't need updating unnecessarily. In
fact all of the ttimp tests - which were just part of the initial
scaffolding - are probably now subsumed by the idris2 tests.
* Add new coverage001 test files
