If an object is sealed, `captureStackTrace` throws an exception.
This will happen for WebAssembly objects.

We now detect this case and fall back to instantiating a dedicated
`js.Error` object.

We now directly use `import("./main.js")` or `require("./main.js")`
rather than relying on the compilation scheme of `js.dynamicImport`.

This will allow `ExportLoopback` to work under WebAssembly,
although the initial implementation will not support multiple
modules.

This commit contains the initial implementation of the WebAssembly
backend. This backend is still experimental, in the sense that:

* We may remove it in a future Minor version, if we decide that it
  has a better place elsewhere, and
* Newer minor versions may produce WebAssembly code that requires
  more recent WebAssembly features.

The WebAssembly backend silently ignores `@JSExport` and
`@JSExportAll` annotations. It is otherwise supposed to support
the full Scala.js language semantics.

Currently, the backend only supports some configurations of the
linker. It requires:

* No optimizer,
* Unchecked semantics for undefined behaviors,
* Strict floats, and
* ES modules.

Some of those will be relaxed in the future, definitely including
the first two.

Co-authored-by: Rikito Taniguchi <[email protected]>

We can rely on the `ToNumber(v)` conversions performed by
`ToWebAssemblyValue` instead of doing `null` handling ourselves.

Also reorder Instructions by opcode.

Except using a typeclass for the block types in `FunctionBuilder`.

We still need an overload of each method for the empty block type,
but the other ones disappear.

Also, we cannot remove the boilerplate from the `switch` overloads,
since `switch` ultimately wants a pair of `FunctionType`s so that
it can decompose them. It could not do that with `BlockType`s.

If a class or interface does not exist but it is mentioned in a
type, it means it had no instances. Therefore, we can transform it
to `nullref`.

…ads.

Discovered while adding checked behaviors for CCEs.

This allows to get rid of the pre-registration of strings used by
module initializers.

And simplify its internals to remove code duplication.