Add a canonical approach for ingesting a raw stream as integer64

[MichaelChirico]

Dedicated issue for #39 / #116 which requested this through as.integer64.raw().

I think we can do this by adding yet-another hijack:

readBin <- function (con, what, n = 1L, size = NA_integer_, signed = TRUE, endian = .Platform$endian) {
  x <- base::readBin(...)
  if (is.integer64(what)) class(x) = 'integer64'
  x  
}

cc @hadley @coolbutuseless

[hadley]

I'd prefer to draw from what most other languages do and provide a dedicated parseInt64BE/parseInt64LE (and maybe UInit too?).

(I'm not sure what the correct prefix here is. read doesn't feel quite right to because it's usually about reading complete files from disk/a connection. But parse() isn't quite right either because there isn't really any parsing going on here.)