We can safely assume that deserialize is enabled for all output tests.
Supporting other modes of operation has low utility.
This change also adds an option to skip the explicit types for WGSL output.
The original pointer access test used SPIR-V for its input because WGSL didn't
have a working pointer indirection operator at the time. Now that it does, we
can just write this test in WGSL directly.
Fixes#1432.
Treat expressions in `Function::named_expressions` like WGSL `let` declarations,
assuming that the Load Rule was applied to the rhs of the declaration, meaning
that their values are always `Indirection::Ordinary`.
Split `write_expr_plain_form` out from `write_expr_with_indirection`, to clean
up the parenthesis generation: no more `opened_paren` variable, just function
calls. This makes the early return for named expressions neater.
Fixes#1382.