os: add `uname` method
os: add `uname` method

This will allow us to replace code like

    os = if OS.mac?
      "Darwin"
    else
      "Linux"
    end

with a call to `OS.uname`. Doing

    rg '= (if )?OS\.(mac|linux)\?'

returns about 70 matches, so there are probably at least a handful of
formulae that could be simplified by this.

nandub/brew,EricFromCanada/brew,nandub/brew,JCount/brew,JCount/brew,Homebrew/brew,JCount/brew,EricFromCanada/brew,MikeMcQuaid/brew,EricFromCanada/brew,nandub/brew,MikeMcQuaid/brew,Homebrew/brew,MikeMcQuaid/brew,JCount/brew,MikeMcQuaid/brew,Homebrew/brew,Homebrew/brew,nandub/brew,EricFromCanada/brew