Rename core types for clarity

[grjte]

This PR addresses the following issues that have come up while working towards support for functions and evaluator functions:

1. lack of distinction between a binding and accessing a binding (e.g. in a function declaration, we need to parse bindings, but in a function call we want to parse accesses of bindings)
2. many structs are named in a format like _Type (e.g. VariableType), but actually they do not identify types in the language. Instead, they identify different expression variations for bindings (e.g. the value of a variable binding can be a single expression, a vector of expressions, a matrix of expressions, etc). This became confusing when implementing functions, as functions actually have types.

The first point is addressed by renaming Constant to ConstantBinding and Variable to VariableBinding.
The second point is addressed by replacing "Type" with "ValueExpr" (e.g. VariableType --> VariableValueExpr), although I'm open to other names. for this

[bobbinth]

Looks good! Thank you!