WIP: checked integer conversions

This is not done yet but ready for discussion at least.

1. fixes #3759, making arithmetic type-preserving
1. fixes #5413, checking integer truncations
1. also checks signed<->unsigned conversions

(1) was simple and did not really break much.
(2) was a bit more breaking, but not too bad and leads to clearer code in most cases.
(3) was kind of a disaster and seems to be very annoying.

The problem with (3) is that interpreting a 2's complement integer as unsigned is mathematically useful. It also comes up a lot in hashing, random number generation, and when you just want to write a byte to an IO stream.

I'm not sure yet how to rewrite https://github.com/JuliaLang/julia/blob/ec607da4d3b25663487dd8c5e773d661b6bb6054/base/random.jl#L209 with checked integer conversions. We need to decide how to handle cases like this.

I think adding these checks is clearly the right thing, but I was surprised at how many legitimate uses of "unsafe" integer conversion there were. I added methods to `itrunc` for unsafely truncating integers, but this only handles converting from larger to smaller. We need to figure out what other functions might be needed.
See the comment right above. LLVM's checked multiply is broken for 8-bit.