unsafe_string should accept both UInt8 and Int8 pointer type

[r9y9]

As Julia master does:

julia> methods(unsafe_string)
#3 methods for generic function "unsafe_string":
unsafe_string(s::Cstring) at c.jl:69
unsafe_string(p::Union{Ptr{Int8},Ptr{UInt8}}) at strings/basic.jl:56
unsafe_string(p::Union{Ptr{Int8},Ptr{UInt8}}, len::Integer) at strings/basic.jl:52

julia> versioninfo()
Julia Version 0.5.0-dev+5063
Commit 8ecd657* (2016-07-01 04:21 UTC)
Platform Info:
  System: Darwin (x86_64-apple-darwin15.5.0)
  CPU: Intel(R) Core(TM) i5-4258U CPU @ 2.40GHz
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.7.1 (ORCJIT, haswell)

Without this, e.g. the following doesn't work (on v0.4.5 at least):

using Compat
unsafe_string(ccall((:getenv, "libc"), Ptr{Cchar}, (Ptr{UInt8},),
            Compat.unsafe_convert(Ptr{UInt8}, "HOME")))

LoadError: MethodError: `unsafe_string` has no method matching unsafe_string(::Ptr{Int8})
while loading In[28], in expression starting on line 1

when Cchar is aliased to Int8.

[stevengj]

You need @compat to use Union{...}, since that syntax won't work on 0.3. Also, unsafe_wrap needs the same fix.

I think #245 should supersede this.