For bootstrapping purposes, this commit does not remove all uses of
the keyword "pure" -- doing so would cause the compiler to no longer
bootstrap due to some syntax extensions ("deriving" in particular).
Instead, it makes the compiler ignore "pure". Post-snapshot, we can
remove "pure" from the language.
There are quite a few (~100) borrow check errors that were essentially
all the result of mutable fields or partial borrows of `@mut`. Per
discussions with Niko I think we want to allow partial borrows of
`@mut` but detect obvious footguns. We should also improve the error
message when `@mut` is erroneously reborrowed.
1. In the first case, the previous code was failing during type inference
due to mismatched structure. Fix is to use the X structure at both
points in the code.
2. In the second case, a naive transcription that subsitutes *nothing*
in for the omitted statements signified by "..." will actually
compile without an error. Furthermore, any pure code could also be
substituted for the ellipsis and the code would compile (as the
text already states). So to make the example more illustrative, it
would be better to include an impure callback, which makes the
potential for aliasing immediately obvious to the reader.
