Improve type repertoire about futhark HOT 6 CLOSED

Note that all of this requires fairly minor changes to the compiler.

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For a start, I changed the interpreter to use 32-bit signed integers internally.

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real has already become float32 and float64. We could easily add float16, which I believe is supported by some GPUs. I think byte is a bad name, and we should use uint8 or word8 instead. As I recall, LLVM uses i32 for a signed 32-bit integer, and u32 for an unsigned 32-bit integer. This is also an okay name, but perhaps the succintness is excessive.

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I will do this soon, as it is necessary to support some benchmarks (like Hashcat from Accelerate). For the internal language, I will most likely copy the LLVM type system: http://nondot.org/~sabre/LLVMNotes/TypeSystemChanges.txt and http://llvm.org/docs/LangRef.html#first-class-types

One interesting feature of the LLVM type system is that there is no distinction between unsigned and signed integer types. In LLVM, you use distinct instructions (like sdiv versus udiv) instead, which is really what you want in a IL anyway.

The external language will have distinguished signed/unsigned types, of course.

The only thing I need to figure out is how to express casts/conversions between the different types. There should of course be the three usual rounding functions from any floating point type to any integer type (and back), but we also need conversion functions between the integers - both sign-extending and zero-extending. And of course, these shouldn't really be "functions" in the internal language, but built-in constructs. I think they should look like functions in the external language, though.

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Also, like C and LLVM (kind of), signed overflow will be undefined. This is mostly to help with range analysis, where x+1>x is a very nice property to have.

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I did part of this. The only thing missing is to add unsigned types. I have come to the conclusion that I do not want automatic type coercion/conversion - it will be more verbose, but you will know what's going on.

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