Consolidate treatment of strictness in parser

Currently GHC's treatment of binding strictness in the Haskell parser and AST is the result of a long, winding evolution. Since D3670 we have tracked strictness of variable bindings (e.g. !x = ...) in the SrcStrictness in FunRhs (see Note [Varieties of bindings]). This is parsed by ``. However, explicitly lazy bindings (e.g. \~x = ...) are parsed via a completely different means (see the rule starting with `~` in `aexp`) and represented as `ELazyPat` until being mangled into a pattern in `RdrHsSyn`. It seems to me like these two paths should be more symmetric.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
Test case
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CC	alanz
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