Adopt a more robust mechanism to serialize TypeSignature parameters

[tdcmeehan]

Right now, if you attempt to deserialize a TypeSignature of the form DECIMAL(a,b), a type signature is returned with two parameters: a, and b. These parameters are recognized as types.

DECIMAL(1,2) does not have any issue: because the input is numeric, we correctly interpret the parameters to be constant longs. But because a and b could represent both a user-supplied built in Type, and a variable, Presto simply chooses the type indescriminately.

In the code, you can specify that a and b are variables by using a constructor argument. However, it would be convenient to define inline that the parameters are actually variables, not types.

I propose the following syntax modeled on Scala pattern matching:

Decimal($_,$_) - there are two anonymous variables (the variable names do not matter and are anonymous)
Decimal($a,$a) - there is one named variable used in two different places
Decimal(BIGINT,VARCHAR) - there are two types, BIGINT and VARCHAR