Fix a bug that encode() and decode() are not re-entrant

[gfx]

close #195

$ npx mocha test/reuse-instances-with-extensions.test.ts


  reuse instances with extensions
    1) should encode and decode a bigint


  0 passing (5ms)
  1 failing

  1) reuse instances with extensions
       should encode and decode a bigint:
     RangeError: Extra 6 of 9 byte(s) found at buffer[3]
      at Decoder.createExtraByteError (src/Decoder.ts:274:12)
      at Decoder.decode (src/Decoder.ts:287:18)
      at Context.<anonymous> (test/reuse-instances-with-extensions.test.ts:38:26)
      at processImmediate (node:internal/timers:491:21)