This also tightens the means of redeclaration of a variable by proxy,
since we now have a way of knowing how a variable was initially
declared, we can check if it was declared using `let` or `const` and
not tolerate redeclaration like we did previously.
Previously, we were assuming all declared variables were bound to a
block scope, now, with the addition of declaration types, we can bind
a variable to a block scope using `let`, or a function scope (the scope
of the inner-most enclosing function of a `var` declaration) using
`var`.
It's now possible to assign expressions to variables. The variables are
put into the current scope of the interpreter.
Variable lookup follows the scope chain, ending in the global object.
Objects can now be allocated via the interpreter's heap. Objects that
are allocated in this way will need to be provably reachable from at
least one of the known object graph roots.
The roots are currently determined by Heap::collect_roots().
Anything that wants be collectable garbage should inherit from Cell,
the fundamental atom of the GC heap.
This is pretty neat! :^)
I always tell people to start building things by working on the thing
that seems the most interesting right now. The most interesting thing
here was an AST + simple interpreter, so that's where we start!
There is no lexer or parser yet, we build an AST directly and then
execute it in the interpreter, producing a return value.
This seems like the start of something interesting. :^)
