v2.0.26-RC3: whitelist support

- support for local whitelist (`whiteList`-property in the nodes-config used when configuring)
   ```js
  {"nodes":{
    "0x1":{
      "whiteList":["0x123455","0x1234556"]
    }
  }}
   ```
- support for contract-based whitelist (`whiteListContract`-property in the nodes-config used when configuring)
   ```js
  {"nodes":{
    "0x1":{
      "whiteListContract":"0x123455"
    }
  }}
   ```
- support for single_chain-instances (`in3_for_chain()`)
- cmake-option to reduce size of the binary for about 37kB, but slower ec-operations.  (`-DUSE_PRECOMPUTED_EC=true`)