Replace the `struct EscapeInfo` with a simpler API, just consisting of methods of `ProjectedValue` and `Value`:
* `isEscaping()`
* `isAddressEscaping()`
* `visit()`
* `visitAddress()`
If there are no more than 2 elements in the cache, we can avoid using the `cache` Dictionary, which avoids memory allocations.
Fortunately this is the common case by far (about 97% of all walker invocations).
* "merge" the `Path` and `State` in WalkUtils into a single `WalkingPath`. This makes it simpler for clients to configure a path and additional state variables. EscapeInfo now defines `EscapePath` which includes the projection path and EscapeInfo's specific state variables.
* Make the `WalkerCache` part of the WalkUtils, so that not all clients have to re-implement it.
* Rename `walkDownResults` -> `walkDownAllResults` and `walkUpOperands` -> `walkUpAllOperands` and make these functions client configurable.
Introduces a set of protocols useful to perform def-use and use-def
traversals to find uses and definitions of values.
This logic was originally baked into `EscapeInfo` directly.
Here we extract it into general utilities, namely:
- `ValueDefUseWalker`: visit uses of a value walking down value-value projections/constructions.
- `AddressDefUseWalker`: visit uses of an address walking down addr-addr projections/constructions.
- `ValueUseDefWalker`: visit definitions of a value walking up value-value projections/constructions.
- `AddressUseDefWalker`: visit definitions of an address walking up addr-addr projections/constructions.
These utilities can then be used in other passes or to create
new utilities by composing them. For example to find a definition
passing through both address projections and value extractions,
it's enough to implement a visitor conforming to both
`AddressUseDefWalker` and `ValueUseDefWalker`.
