Remove the restriction of nbytes in unsafe_read (#104)

* Remove the restriction of nbytes in unsafe_read

The original restriction of `Int` will cause errors in some cases. Replaced with `Integer` instead. I think this also aligns with the signatures in Base.
 
* Remove redundant return signature

Author: findmyway

src/io.jl

@@ -11,7 +11,8 @@ This function is similar to `Base.unsafe_read` but is different in some points:
 - It returns the number of bytes written to `output`.
 - It does not block if there are buffered data in `input`.
 """
-function unsafe_read(input::IO, output::Ptr{UInt8}, nbytes::Int)::Int
+function unsafe_read(input::IO, output::Ptr{UInt8}, nbytes::Integer)
+    nbytes = convert(UInt, nbytes)
     p = output
     navail = bytesavailable(input)
     if navail == 0 && nbytes > 0 && !eof(input)