Fix Valgrind issues in 'stream' examples

[jiribenes]

In #705, we disabled two of the tests because of Valgrind problems:

effekt/effekt/jvm/src/test/scala/effekt/StdlibTests.scala

Lines 49 to 50 in c6e2202

	examplesDir / "stdlib" / "stream" / "characters.effekt",
	examplesDir / "stdlib" / "stream" / "fuse_newlines.effekt",

It would be nice to get them working again. :)

[marvinborner]

Both tests leak bytearrays. This comes down to a similar issue with array allocations that we've had for a long time. Several wrapper functions call the semi-internal allocation function (...array_new) directly, but the allocated objects never get erased.

That's why:

import bytearray

// leaks 17 byte
def main() = {
  bytearray::allocate(1)
  ()
}

as well as

import array

// leaks 40 byte
def main() = {
  val arr: Array[Int] = array::allocate(1)
  ()
}

I'm still not sure how we want to fix this in the long term. We must make it a lot clearer which (C) functions are internal and could leak, so we don't use them accidentally.