Reduce usage of `let GeoType(a) = b`

[nyurik]

Constructs such as

let &geo_types::LineString(ref g_points) = g_line;
w_lines.push(g_points_to_w_linestring(g_points));

cannot be easily ported if we introduce 3D/M coordinates. This PR changes them to the direct tuple access. Eventually we may want to create dedicated tuple member accessors instead.

w_lines.push(g_points_to_w_linestring(&g_line.0));

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