syntax: convert ast_map to use a SmallIntMap.

NodeIds are sequential integers starting at zero, so we can achieve some
memory savings by just storing the items all in a line in a vector.

The occupancy for typical crates seems to be 75-80%, so we're already
more efficient than a HashMap (maximum occupancy 75%), not even counting
the extra book-keeping that HashMap does.
auto merge of #11616 : huonw/rust/ast_map, r=pnkfelix

NodeIds are sequential integers starting at zero, so we can achieve some
memory savings by just storing the items all in a line in a vector.

The occupancy for typical crates seems to be 75-80%, so we're already
more efficient than a HashMap (maximum occupancy 75%), not even counting
the extra book-keeping that HashMap does.