improve class=path styling (#1450)

This PR addresses that by:

* rendering oneway arrows on more than `subclass=cycleway`
* less opacity for path/footway casing
* adding the thin black casing to bridges
* adding `subclass=steps` & `subclass=bridleway` rendering
* render `bicycle=designated` as `subclass=cycleway` and `horse=designated` as `subclass=bridleway` (bike taking priority)
* improved tunnel rendering with double casing & adding main line
* made casing min zoom consistent across subclasses

All of these changes were made to mimic OSM Carto.

style/icons/oneway-path.svg

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   version="1.1"
+   width="12"
+   height="5"
+   id="svg2">
+  <defs
+     id="defs4" />
+  <metadata
+     id="metadata7">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <path
+     d="M 7.101959,2.9592164 H 0.03050935 V 2.074617 H 7.101959 7.178542 V 1.9921537 0.02381691 L 12.02987,2.5169084 7.1785457,5.0100108 V 3.0416796 2.9592164 Z"
+     fill="#000080"
+     id="path833"
+     style="stroke-width:0.794704" />
+</svg>