Report a domain error when const folding ** yields a complex number (#228)

[zimri-leisher]

Fixes #228.

Const-folding ** calls Python float.__pow__, which returns a complex number
for a negative base and a fractional exponent instead of raising. The folder
only expects int, float, Decimal or bool, so it died on assert False, folded_value in CalculateConstExprValues.visit_AstBinaryOp. The Decimal path
for the same expression raises decimal.InvalidOperation and is already
reported as Domain error; this makes the float path agree.

Red/green: the two new sequences (F64(-8.0) ** 0.5 and (-2.0) ** F64(0.5))
crash the compiler with an AssertionError before the change and report
Domain error after. The existing (-1) ** 0.5 test covers the Decimal path
and passes throughout.

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