This PR adds the directory `Meta/DiscrTree` and reorganizes the code
into different files. Motivation: we are going to have new functions for
retrieving simplification theorems for the new structural simplifier.
This PR removes the old ElimDeadBranches pass and shifts the new one
past lambda lifting.
The reason for dropping the old one is its general unsoundness and the
fact that we want to do refactorings on the IR part. The reason for
shifting the current pass past lambda lifting, is that its analysis is
imprecise in the presence of local function symbols. I experimented with
the exact placement for a while and it seems like it is optimal here.
Overall we observe a slight regression in the amount of C code
generated, likely because we don't propagate information into lambdas
before lifting them anymore. But generally measure a slight performance
improvement in general.
This PR replaces `MatcherInfo.numAltParams` with a more detailed data
structure that allows us, in particular, to distinguish between an
alternative for a constructor with a `Unit` field and the alternative
for a nullary constructor, where an artificial `Unit` argument is
introduced.
This PR replaces all usages of `[:]` slice notation in `src` with the
new `[...]` notation in production code, tests and comments. The
underlying implementation of the `Subarray` functions stays the same.
Notation cheat sheet:
* `*...*` is the doubly-unbounded range.
* `*...a` or `*...<a` contains all elements that are less than `a`.
* `*...=a` contains all elements that are less than or equal to `a`.
* `a...*` contains all elements that are greater than or equal to `a`.
* `a...b` or `a...<b` contains all elements that are greater than or
equal to `a` and less than `b`.
* `a...=b` contains all elements that are greater than or equal to `a`
and less than or equal to `b`.
* `a<...*` contains all elements that are greater than `a`.
* `a<...b` or `a<...<b` contains all elements that are greater than `a`
and less than `b`.
* `a<...=b` contains all elements that are greater than `a` and less
than or equal to `b`.
Benchmarks have shown that importing the iterator-backed parts of the
polymorphic slice library in `Init` impacts build performance. This PR
avoids this problem by separating those parts of the library that do not
rely on iterators from those those that do. Whereever the new slice
notation is used, only the iterator-independent files are imported.
This PR ensures the new code generator produces code for `opaque`
definitions that are not tagged as `@[extern]`.
Remark: This is the behavior of the old code generator.
The previous implementation had a few issues:
- Function (and join point) declarations were being inserted into two different hashmaps.
- `borrow` information was not available for parameters.
- No proper erase functions.
