Experimental: add getErrorForest

[PoldarnKJ]

Add getErrorForest for customize error report.

   const decoder = _.type({
        a: _.string
      })
      assert.deepStrictEqual(
        pipe(decoder.decode({ c: [1] }), E.mapLeft(_.getErrorForest)),
        E.left([
          {
            value: {
              _tag: 'Key',
              key: 'a',
              kind: 'required'
            },
            forest: [
              {
                value: {
                  _tag: 'Leaf',
                  error: 'string',
                  actual: undefined
                },
                forest: []
              }
            ]
          }
        ])
      )
    })