Implementation of lint checking.
The lint checking is mostly consolidated into one pass which runs
after all other analyses. Throughout compilation, lint warnings
can be added via the
add_lint method on the Session structure. This
requires a span and an ID of the node that the lint is being added to. The
lint isn’t actually emitted at that time because it is unknown what the
actual lint level at that location is.
To actually emit lint warnings/errors, a separate pass is used. A context keeps track of the current state of all lint levels. Upon entering a node of the ast which can modify the lint settings, the previous lint state is pushed onto a stack and the ast is then recursed upon. As the ast is traversed, this keeps track of the current lint level for all lint attributes.