Log Message: 
Added easy calls to disable/enable collections of functions or
individual functions in the current context. So you can say
Parser::Context::Functions::Disable('Trig');
to disallow the use of trig functions, or
Parser::Context::Functions::Disable('Trig','Numeric');
Parser::Context::Functions::Enable('sqrt');
to disable trig and numeric functions (like log, exp, abs, etc) but
allow sqrt.
Note that disabling sqrt() does not disable x^(1/2) (this must be done
through disabling the ^ and ** operators), and disabling abs() does
not disable x (which must be done by removing the '' definition
from the Context's parens list).
