Various tests fixes - Method <-> Function
Fixed placeholder guard ordering, Guards

Switched from using push/pop to unshift/shift to add guards to the front
of the list to respect the LIFO ordering of guards

This also includes a fix for the Number, Function etcfunctions, which
now properly throw the "typecheck failed..." exception messages
Added Dot Looking up Mutable Exception

"Cannot look up mutable field using dot or bracket"
Fixed Capital letter guards to return booleans

Number, Function etc now are identical in functionality to is-number, is-function...
Counting Nodes

count-nodes.arr contains the function count-nodes(ast) which walks the
ast tree (A.Expr) and counts the total number of nodes in the tree.

A useful tester function is count-in(str) which parses the string as a
pyret program and counts the number of Expr nodes.