Support Builtin Function

[at15]

In order to perform things like query from database, we either define special syntax or use a function

• query { select from a where value < c }
• query("select from ${a} where value < ${c} and time > ${t}")

I prefer the function one (at least now), also there will be function like abs, avg in normal SQL languages.
Introduce function requires some improvement in type checking #4 ,

• built in function symbols should be pre loaded into symbol table
• can't define variable that use the same ID as built in functions
• check variable type when build the AST
• logic for ExpStat in evaluation, it is currently empty