Rollup merge of #35874 - CensoredUsername:stmt_let_typed_fix, r=Manishearth
Rollup merge of #35874 - CensoredUsername:stmt_let_typed_fix, r=Manishearth

 This commit makes the return type of AstBuilder.stmt_let_typed match the return type of other AstBuilder.stmt* functions. This avoids unnecessary boxing/unboxing whenever Stmt's are stored in a Vec, which is the default use case.nnThis is a potentially plugin breaking change.

GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust