plperl update from Andrew Dunstan, deriving (I believe) from Command Prompt's

plperlNG. Review and minor cleanup/improvements by Joe Conway.

Summary of new functionality:
- Shared data space and namespace. There is a new global variable %_SHARED
  that functions can use to store and save data between invocations of a
  function, or between different functions. Also, all trusted plperl function
  now share a common Safe container (this is an optimization, also), which
  they can use for storing non-lexical variables, functions, etc.
- Triggers are now supported
- Records can now be returned (as a hash reference)
- Sets of records can now be returned (as a reference to an array of hash
  references).
- New function spi_exec_query() provided for performing db functions or
  getting data from db.
- Optimization for counting hash keys (Abhijit Menon-Sen)
- Allow return of 'record' and 'setof record'