…> T (#3759)

the sysimg builds, but still need to:
  - check same-size signed<->unsigned conversion
  - make tests pass

…able arithmetic

this would be a bit cleaner without the code in intfuncs that dispatches
on Unsigned.

signed() and unsigned() are convenient for testing bit patterns in the
test suite. for now replace with assigned() and asunsigned().

uint8(x) is convenient for moving bytes around, so it remains unchecked.
we should perhaps rename this to byte(), and either eliminate uint8()
entirely, or make it checked.

…nt_trunc

Conflicts:
	base/number.jl

…nt_trunc

Conflicts:
	base/intfuncs.jl

a few fixes in base for the convert and arithmetic changes

get all tests passing

type-preserving arithmetic.

allow reducedim(+, A, 2), which previously gave
```
ERROR: `reducedim_init` has no method matching reducedim_init(::IdFun, ::Function, ::Array{Uint8,2}, ::Int64)
 in reducedim at reducedim.jl:217
```

…nt_trunc

Conflicts:
	base/reduce.jl

also fix sum_kbn on empty input

…nt_trunc

…nt_trunc

Conflicts:
	base/reduce.jl

…nt_trunc