add readFileString, writeFileString, appendFileString

add readFileString, writeFileString, appendFileString

Exported by Common, so they will be available everywhere.
These are the same as readFile, writeFile, appendFile.
But have two benefits:

* They take OsPath, so using them avoids converting back and forth
  unncessarily.
* They use the close-on-exec flag so can't leak FDs to child processes.
  Unlike the standard Haskell versions which unfortunately have that
  wart currently. (I do hope the standard versions get fixed
  eventually.)

Sponsored-by: the NIH-funded NICEMAN (ReproNim TR&D3) project