This change checks in an enormously rudimentary interpreter. There is
a lot left to do, as evidenced by the countless TODOs scattered throughout
pkg/compiler/eval/eval.go. Nevertheless, the scaffolding and some important
pieces are included in this change.
In particular, to evaluate a package, we must locate its entrypoint and then,
using the results of binding, systematically walk the full AST. As we do
so, we will assert that aspects of the AST match the expected shape,
including symbols and their types, and produce value objects for expressions.
An *unwind structure is used for returns, throws, breaks, and continues (both
labeled and unlabeled). Each statement or expression visitation may optionally
return one and its presence indicates that control flow transfer is occurring.
The visitation logic then needs to propagate this; usually just by bailing out
of the current context immediately, but sometimes -- as is the case with
TryCatchBlock statements, for instance -- the unwind is manipulated in more
"interesting" ways.
An *Object structure is used for expressions yielding values. This is a
"runtime object" in our system and is comprised of three things: 1) a Type
(essentially its v-table), 2) an optional data pointer (for tagged primitives),
and 3) an optional bag of properties (for complex object property values).
I am still on the fence about whether to unify the data representations.
The hokiest aspect of this change is the scoping and value management. I am
trying to avoid needing to implement any sort of "garbage collection", which
means our bag-of-values approach will not work. Instead, we will need to
arrange for scopes to be erected and discarded in the correct order during
evaluation. I will probably tackle that next, along with fleshing out the
many missing statement and expression cases (...and tests, of course).
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