postgres-lambda-diff

- use "bool" rather than "int" for boolean variables

- use "PLy_malloc" rather than "malloc" in two places

- define "PLy_strdup", and use it rather than malloc() + strcpy() in
  two places (which should have been memcpy(), anyway).

- remove a bunch of redundant parentheses from expressions that do not
  need the parentheses for code clarity