As in non-local scopes, local types and functions are generally
visible anywhere within enclosing braces, and it is up to capture
analysis to determine whether these entities attempt to capture an
entity that isn't guaranteed to be available.
Once top-level code is introduced into a source file, turn the source
file itself into a continuation, so that subsequent declarations have
proper scope information. This means we get proper scope information
for, e.g., pattern bindings and guard statements in top-level code, so
that a 'guard' at top level is a parent of all subsequent code,
including (e.g.) function declarations that might capture variables
introduced by the guard.
Continuations are now handled by more explicitly passing them down
from parent nodes to the child nodes that can accept them, and the
source file itself is modeled as a continuation as soon as the first
top-level code declaration is encountered.
