rangeTest: Fix signed-ness of types

[bgamari]

On some architectures types like CChar are signed whereas the use previously
assumed they were unsigned. Fix this by relying on Bounded instances where
possible.

[bgamari]

Ping?

[bgamari]

How's this look?

[rrnewton]

Looks good for me, very sorry for delay.

[thomie]

@rrnewton Thanks for merging all the patches, and the release.

@bgamari There is still one small typo:

(undefined:: CWchar)

should be changed to:

(undefined:: CSigAtomic)

On the line that starts with:

checkBounds "CSigAtomic R"

[bgamari]

@thomie very good catch, sir.