Wasm-split generally assumes that calls to secondary functions made
before the secondary module has been loaded and instantiated should go
to imported placeholder functions that can be responsible for loading
the secondary module and forwarding the call to the loaded function.
That scheme makes the loading entirely transparent from the
application's point of view, which is not always a good thing. Other
schemes would make it impossible for a secondary function to be called
before the secondary module has been explicitly loaded, in which case
the placeholder functions would never be called. To improve code size
and simplify instantiation under these schemes, add a new
`--no-placeholders` option that skips adding imported placeholder
functions.

Add a mode that splits a module into arbitrarily many parts based on a
simple manifest file. This is currently implemented by splitting out one
module at a time in a loop, but this could change in the future if
splitting out all the modules at once would improve the quality of the
output.

Rather than analyze what module elements from the primary module a
secondary module will need, the splitting logic conservatively imports
all module elements from the primary module into the secondary module.
Run RemoveUnusedElements on the secondary module to remove any of these
imports that happen to be unnecessary. Leave a TODO mentioning the
possibility of being more selective about which module elements get
exported to reduce code size in the primary module, too.