replace typeof(x) <: with x isa

[pepijndevos]

@oscardssmith was telling me there is a lot of this pattern all over SciML, and that the Julia compiler can better reason about isa to allow better inference in if statements and such.

It seems like a fairly mechanical no-brainer accomplished with the following shell command in case this PR becomes stale or you'd like to apply the same change elsewhere.

git ls-files -z | xargs -0 sed -i -e 's/typeof(\([^)]*\)) <:/\1 isa/g'