[let_chains, 2/6] Introduce `Let(..)` in AST, remove IfLet + WhileLet and parse let chains
Here we remove `ast::ExprKind::{IfLet, WhileLet}` and introduce `ast::ExprKind::Let`.
Moreover, we also:
+ connect the parsing logic for let chains
+ introduce the feature gate
+ rewire HIR lowering a bit.
However, this does not connect the new syntax to semantics in HIR.
That will be the subject of a subsequent PR.
Per https://github.com/rust-lang/rust/issues/53667#issuecomment-471583239.
Next step after https://github.com/rust-lang/rust/pull/59288.
cc @Manishearth re. Clippy.
r? @oli-obk
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| .. | ||
| tt | ||
| base.rs | ||
| build.rs | ||
| derive.rs | ||
| expand.rs | ||
| placeholders.rs | ||
| source_util.rs | ||