feature: stages

stages: separate scanning and parsing, allowing `v`, `vn`, `vt` lines to
be mapped to line numbers and store line data (once).

first stage parsing: parse face data and validate vertices.

TODO:
* improve validation (detect partial normals/texcoords);
* maybe improve storage of triangle line number instead of storing line
  number for each vertex;

src/trim.cpp

@@ -0,0 +1,16 @@
+#include "trim.hpp"
+
+namespace wavefront {
+    std::string_view trim(const std::string_view &source) {
+        auto start = source.data();
+        // size is the size of buffer, data() + size() is the first available byte after the string.
+        auto end = source.data() + source.size() - 1;
+        while (start < end && std::isspace(*start)) {
+            ++start;
+        }
+        while (start < end && (std::isspace(*end) || *end == 0)) {
+            --end;
+        }
+        return std::string_view(start, end - start + 1);
+    }
+}