This PR sets up the new integrated test/bench suite. It then migrates
all benchmarks and some related tests to the new suite. There's also
some documentation and some linting.
For now, a lot of the old tests are left alone so this PR doesn't become
even larger than it already is. Eventually, all tests should be migrated
to the new suite though so there isn't a confusing mix of two systems.
With the recent unification of server and cmdline processing,
`IO.Process` tests that previously broke the server because they
directly wrote to stdout are now flaky on the cmdline because
elaboration and reporting are happening in separate threads. By removing
direct writes to stdout, the race condition is removed and the file can
actually be edited in the language server as well again.
After this commit, we have to use an explicit `discard` in code such as
```
def g (x : Nat) : IO Nat := ...
def f (x : Nat) : IO Unit := do
discard <| g x -- type error without the `discard`
IO.println x
```
Motivation: prevent users from making mistakes such as
```
def f (xs : Array Nat) : IO Unit := do
xs.set! 0 1
IO.println xs
```
when they meant to write
```
def f (xs : Array Nat) : IO Unit := do
let xs := xs.set! 0 1
IO.println xs
```
