core: move contents to mpvcore (1/2)

core is used in many unix systems for core dumps. For that reason some tools
work under the assumption that the file is indeed a core dump (for example
autoconf does this).

This commit just renames the files. The following one will change all the
includes to fix compilation. This is done this way because git has a easier
time tracing file changes if there is a pure rename commit.
core: move contents to mpvcore (2/2)

Followup commit. Fixes all the files references.