[Experimental] Print descriptors as vtable fields

[tlively]

Also print exact references as inexact references. This prints modules
using custom descriptors as if they were not using custom descriptors,
making it easier to identify optimization regressions by diffing modules
before and after enabling custom descriptors.

[tlively]

cc @kripken in case this is useful for you. This is especially helpful in combination with -g --preserve-type-order. You can also use this command to remove numerical type names Binaryen emits, since differences in the numbers names are not typically interesting: sed -i 's/\$[0-9]\+/\$/g' disassembled.wast