Support default value for all types and 'default' operator

[tmteam]

every type has it's default value

f(a,b) = 
   if (a>b) a
   if (b>a) b
   else default

default(any) = none # depends on nullability support.
default(int) = 0
default(real) = 0.0
default(bool) = false
default(text) = ""
default( array of t) = [ ]
default {field1, field 2} = { field1 = default, field2 = default}
default rule(T,T):R =    rule(T,T):R = default