Lr:var should be restricted to only having strings as their name

[vcanumalla]

Currently, lr:var can be instantiated with symbols or strings as their names. This can cause a number of issues.

In my experience, this manifested in this way.

When I tried to lookup a lr:var that was present in the environment, the corresponding environment entry had a string name rather than a symbol name (which the parser I was using to create the lr:var made) had produced. This strangely resulted in a union-merge error in Rosette, rather than an entry not found error that I would have expected.