JS code generation does not work with single-field structures

[eggrobin]

This was previously spotted in #70, as the example had such a struct, but the fix to that issue was merely to change the example so it did not rely on behaviour not yet implemented.

[cargo-make] INFO - Running Task: diplomat-gen-js
thread 'main' panicked at 'not yet implemented: Receiving structs that don't need a buffer', tool\src\js\conversions.rs:121:25

This is a problem for ICU4X, as the removal of the sign_display option from ICU4XFixedDecimalFormatOptions (as part of unicode-org/icu4x#1267, see unicode-org/icu4x#2025 (comment)) turns it into a single-field struct:

             ICU4XFixedDecimalFormatOptions {
                 grouping_strategy: ICU4XFixedDecimalGroupingStrategy::Auto,
-                sign_display: ICU4XFixedDecimalSignDisplay::Auto,
             }

[Manishearth]

Yeah we'll basically have to special case codegen there, because WASM treats these differently.

[Manishearth]

diplomat/tool/src/js/conversions.rs

Lines 124 to 126 in ce93a1b

	if needs_buffer != ReturnTypeForm::Complex {
	todo!("Receiving structs that don't need a buffer")
	}