Speed up realpath() with dynamically loaded C extension

On systems that support both C compiling and dynamic loading, we can
speed up `realpath()` (where most time in rbenv is spent) by replacing
it with a dynamically loaded bash builtin.

When `make -C src` is called in the project's root,
`libexec/rbenv-realpath.dylib` will be created. If it exists, rbenv will
attempt to load it as a builtin command. If it fails, execution will
fall back to the old `realpath()` shell function.

src/bash.h

@@ -0,0 +1,31 @@
+#ifndef __BASH_H__
+#define __BASH_H__
+
+#define EXECUTION_SUCCESS 0
+#define EXECUTION_FAILURE 1
+#define EX_USAGE 258
+
+#define BUILTIN_ENABLED 1
+
+typedef struct word_desc {
+  char *word;
+  int flags;
+} WORD_DESC;
+
+typedef struct word_list {
+  struct word_list *next;
+  WORD_DESC *word;
+} WORD_LIST;
+
+typedef int sh_builtin_func_t(WORD_LIST *);
+
+struct builtin {
+  char *name;
+  sh_builtin_func_t *function;
+  int flags;
+  char * const *long_doc;
+  const char *short_doc;
+  char *unused;
+};
+
+#endif