Add a new AstVar class that is similar to AstComputedField but is only used in $let contexts.

While AstVar and AstComputedField or structurally similar, they are subtly different. One way they are different is that they are used in different contexts. Another way they are different is that AstVar has a `name` (which must be a valid variable name) whereas AstComputedField has a `path` (which might include dots).