[interpreter] Handle custom sections and annotations (#17)

* [interpreter] Handle custom sections and annotations

Co-authored-by: Yuri Iozzelli <y.iozzelli@gmail.com>

* Fix merge conflict

* Fix lexer priorities

* Fix wast.ml

* Oops

* Update wast.ml

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Co-authored-by: Andreas Rossberg <rossberg@mpi-sws.org>

Author: yuri91