Make List.map_filter(_i) tail-recursive.
Make List.map_filter(_i) tail-recursive.

While the performance gain should go unnoticed in most cases, in some
degenerate situations, e.g. the evar-stressing test-case of bug #4964,
this commit speeds up coq by 10% since most of the time is spent scanning
long lists with most of the elements filtered out.

Note that this commit also changes the scanning order to front-to-back,
which is a bit less surprising (though no code should ever depend on the
scanning order).

amintimany/coq,ejgallego/coq,JasonGross/coq,coq/coq,maximedenes/coq,JasonGross/coq,letouzey/coq-wip,silene/coq,Zimmi48/coq,ejgallego/coq,Ptival/coq,amintimany/coq,maximedenes/coq,coq/coq,JasonGross/coq,william-lawvere/coq,SkySkimmer/coq,coq/coq,Matafou/coq,gares/coq,Zimmi48/coq,letouzey/coq-wip,maximedenes/coq,amintimany/coq,ppedrot/coq,Lionel-Rieg/coq,gares/coq,Zimmi48/coq,letouzey/coq-wip,ejgallego/coq,gares/coq,JasonGross/coq,coq/coq,gares/coq,Ptival/coq,Matafou/coq,ppedrot/coq,ppedrot/coq,maximedenes/coq,Matafou/coq,william-lawvere/coq,SkySkimmer/coq,gares/coq,silene/coq,Lionel-Rieg/coq,william-lawvere/coq,ppedrot/coq,ejgallego/coq,letouzey/coq-wip,SkySkimmer/coq,ppedrot/coq,silene/coq,Zimmi48/coq,letouzey/coq-wip,Ptival/coq,maximedenes/coq,SkySkimmer/coq,Lionel-Rieg/coq