[#671] Better type errors on invalid int casts

Description

Problem: Safe(r) fromIntegral (i.e. intCast) reports a confusing type error "Couldn't match type ‘'False’ with ‘'True’" on invalid conversion.

Solution: Introduce CheckIntSubType constraint synonym, equivalent to IsIntSubType a b ~ 'True, but reporting a human-readable TypeError when IsIntSubType a b ~ 'False.

Related issue(s)

Resolves #671 (closed)

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