Add typeasserts to convert(::Type{Face}, ::Dict)

[topolarity]

This removes a ton of implicit converts from this code, which are easily invalidated.

[tecosaur]

Thanks Cody! I'll admit to having made the assumption that checking x isa Int would have the compiler assume x::Int within the scope of the check.

[topolarity]

That assumption is actually true - the unintuitive part here is that the compiler can't do that for d[key]
(because it doesn't know whether the dictionary is aliased and being secretly modified by another name elsewhere)

It's not uncommon to do:

x = d[key]
if x isa Int ...

for that reason