feat(examples): add a hello project with a library and a consumer

Smallest project that exercises the whole pipeline: one static library and one
executable that links against it.

examples/hello/include/mathx.h

@@ -0,0 +1,12 @@
+#ifndef MATHX_H
+#define MATHX_H
+
+#include <stddef.h>
+
+double vec_dot(const double *a, const double *b, size_t n);
+double vec_norm(const double *a, size_t n);
+double vec_sum(const double *a, size_t n);
+double stats_mean(const double *a, size_t n);
+double stats_stddev(const double *a, size_t n);
+
+#endif