Evaluation failure on unbound variables

Unbound expressions can't currently be converted to terms, since the symbols which bind the variables are immediately evaluated.

```julia
julia> @term(x -> x)
ERROR: UndefVarError: x not defined
Stacktrace:
[1] top-level scope at REPL[2]:1
```

Start by reproducing the @term(x -> x) example from the issue and then inspect the implementation of @term and its handling of unbound expressions. Done means the example converts the expression to a term without raising UndefVarError, with coverage for this behavior added to the relevant test suite.

Written by the indexing model from the issue text.