Go: refactor (#16602)

### Summary

1. update doc
2. refactor route code

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/binding/cpp/stemmer/api.h

@@ -0,0 +1,31 @@
+
+#pragma once
+
+typedef unsigned char symbol;
+
+/* Or replace 'char' above with 'short' for 16 bit characters.
+
+   More precisely, replace 'char' with whatever type guarantees the
+   character width you need. Note however that sizeof(symbol) should divide
+   HEAD, defined in header.h as 2*sizeof(int), without remainder, otherwise
+   there is an alignment problem. In the unlikely event of a problem here,
+   consult Martin Porter.
+
+*/
+
+struct SN_env {
+    symbol *p;
+    int c;
+    int l;
+    int lb;
+    int bra;
+    int ket;
+    symbol **S;
+    int *I;
+    unsigned char *B;
+};
+
+extern struct SN_env *SN_create_env(int S_size, int I_size, int B_size);
+extern void SN_close_env(struct SN_env *z, int S_size);
+
+extern int SN_set_current(struct SN_env *z, int size, const symbol *s);