Pascal's little TODO list

[pascallouisperez]

• The expression interface: 1. no need to make functions exported, 2. Args should return [][]string to be selectors directly (and ePlugin can split on . when going from ComputedBy to expression)
• Rename ComputedBy interface to make it clearer that it can also be used for constrained_by externals
• Address all TODO(pascal) in the code.
• More cases in TestParser_parseType.
• Remove possible infinite recursion in String of *Worksheet, e.g. ws pointing to itself.
• Return struct from DbState helper instead of a 4-tuple.