Solve the annoying `C with AnyRef` problem with contexts

[LPTK]

Sometimes one still encounters annoying errors saying a context should be C with AnyRef instead of just C.
This basically happens when using constructs that return IR[T,{}], such as Const(123) because Const is defined as [T: IRType](v: T): IR[T,{}]. For example:

def foo[C](x: IR[Int, C]): IR[Int, C] = ir"$x + ${Const(123)}"

This code fails to compile with:

Error:(37, 56) type mismatch;
 found   : squid.TestDSL.IR[Int,AnyRef with C]
 required: squid.TestDSL.Predef.IR[Int,C]
    (which expands to)  squid.TestDSL.IR[Int,C]
    def foo[C](x: IR[Int, C]): IR[Int, C] = ir"$x + ${Const(123)}"

[LPTK]

Solved in https://github.com/epfldata/sc/commit/3c4f3fa91f0e336d3f3eed6f2662b2cbb3f2cf47

We now remove AnyRef types from inferred contexts.

Using {} for the empty context is debatable (though it does look better than Any), but at least now it should cause far fewer problems.