Implement VectorAppender of RunEndEncodedVector

[ViggoC]

Describe the enhancement requested

Sub task of #149

[ViggoC]

Hi @lidavidm, I want to discuss a question before we start implementing it. Do you think it's valid for two adjacent runs to have the same value?
e.g.

values	run_ends
1	3
1	5

I know it would be better to merge these two runs into

values	run_ends
1	5

But is it allowed to keep it seperately?

[lidavidm]

As far as I can see, this is not prohibited by the spec, so keeping them is fine 1 2