Implement file:* functions

[skx]

As noted in the previous release notes we should add some file-functions:

• file:stat
  • Return a list of details, size, owner, gid, mode, etc.
  • Add helper (file:stat:size) or similar for each field.
• file:lines
  • Return a list of strings.
• file:read
  • Behave the same as our existing slurp function, which will become an alias.