Add p_value_get() / p_value_get_XXX() accessors

Allow user code to access specific pvalue values, especially when
multiple ptypes are in use.

spec/test_value_accessors.d

@@ -0,0 +1,29 @@
+import testparser;
+import std.stdio;
+
+int main()
+{
+    return 0;
+}
+
+unittest
+{
+    p_token_info_t token_info;
+    string input = "42 f s";
+    p_context_t * context;
+    context = p_context_new(input);
+
+    /* Default ptype value extracted with p_value_get(). */
+    assert(p_lex(context, &token_info) == P_SUCCESS);
+    assert(token_info.token == TOKEN_num);
+    assert(p_value_get(&token_info.pvalue) == 42);
+
+    /* Named ptype values extracted with p_value_get_XXX(). */
+    assert(p_lex(context, &token_info) == P_SUCCESS);
+    assert(token_info.token == TOKEN_flt);
+    assert(p_value_get_float(&token_info.pvalue) == 1.5);
+
+    assert(p_lex(context, &token_info) == P_SUCCESS);
+    assert(token_info.token == TOKEN_str);
+    assert(p_value_get_string(&token_info.pvalue) == "hello");
+}