add typealiases for exact C-compatible types

some more typealias's for consideration in the list that I forgot initially (in decreasing order of my interest level of actually having them added):
```julia
typealias Cfloat Float32
typealias Cdouble Float64
typealias Cstring Ptr{Uint8}
typealias Cargv Ptr{Ptr{Uint8}}
typealias Cptr Ptr{Void}
```