Parse a string 'value' obtained from a request
using a specified 'parser' and save it into an appopriate 'obj' slot
obtained from a view 'field'.

This function returns three values. The first value is true if
'request-slot-value' has successfully been parsed and false
otherwise. If the first value is true, the second value is true if the
input is not empty, and false otherwise. If the second value is true,
the third value is the actual parsed value.

Specialize this function to add custom parsers or to parse given
fields differently.