fix: Force finalizers to explore the graph of objects

[Hywan]

First, this patch provides new Close methods on some “top types”
(Store, Module, Instance etc.). The Close methods are
redundant to the runtime finalizers, but they are useful to force to
destruct a specific type manually before the Go GC collect it.

Note that calling Close will remove the runtime finalizer attached
to the object.

Note that runtime finalizers call the Close method if defined on the
current object.

Second, this patch forces the destructors (now the Close methods) to
destruct the children attached to the current object. Let's see it in
details:

• Module now has two new fields importTypes *importTypes and
  exportTypes *exportTypes. The value is null, except if the
  Imports or Exports methods are called. They will store their
  respective results in their respective fields. The goal of this
  change is twofold:

  1. Avoiding computing the same import and export types multiple
     times each time one of the method is called,

  2. By holding a reference to the private importTypes and
     exportTypes, Module can free them. Before that, the objects
     were orphan and the garbage collector had difficulties to collect
     them.

• Instance now forces to free its Exports,

• Exports now force to free all the items of its exports map
  field. Before this patch, only the _inner C pointer was freed, but
  not the map. I have no idea why the map wasn't collected by the Go
  GC.

Running the script written by @prep in
#262 shows that there is
no more memory leaks.

Fix #262.
Fix #269.

[Hywan]

bors try

[Hywan]

bors try

[bors]

try

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