estack.eclass: Split estack* logic from eutils

estack.eclass: Split estack* logic from eutils

Split the estack_* and related functions from eutils into a dedicated
estack.eclass. Those functions have significant complexity and are not
used frequently, therefore they benefit from having a separate file
and an explicit dedicated maintainer.

The new eclass is implicitly inherited by eutils to preserve
compatibility. However, the inherit will be removed in EAPI 7,
and the ebuilds should switch to using estack directly.

Thanks to Ulrich Müller for doing the research on this.