compiler: rework export index allocation

The current implementation of the module export offset tracking was
inadequate and failed to properly handle larger module dependency
graphs. In order to properly support nested module imports/exports,
the following changes have been introduced:

 - Gather export slots during module compilation and emit corresponding
   export opcodes as one contiguous block at the end of the module
   function body, right before the final return. This ensures that
   interleaved imports of other modules do not place foreign exports
   between our module exports.

 - Track the number of program wide allocated export slots in order
   to derive per-module-source offsets for the global VM export list.

 - Derive import opcode source index from the module source export
   offset and the index of the requested name within the module source
   export name list.

 - Improve error reporting for circular module imports.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

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