private Token peek() {
      if (iToken < typeInfoTokens.size()) {
        return typeInfoTokens.get(iToken);
      } else {
        return null;
      }
    }
Tokenize the typeInfoString. The rule is simple: all consecutive
alphadigits and '_', '.' are in one token, and all other characters are
one character per token.

tokenize("map<int,string>") should return
["map","<","int",",","string",">"]

Note that we add '$' in new Calcite return path. As '$' will not appear
in any type in Hive, it is safe to do so.
serde/src/java/org/apache/hadoop/hive/serde2/typeinfo/TypeInfoUtils.java