Make print and summary functions for session object

Presently printing a `session` displays the following:
```R
> ses
<environment: 0x5583ffd49208>
attr(,"class")
[1] "session"     "environment"
```
and calling `summary()` raises different errors:
```R
> summary(ses)
Error in order(var_counts, decreasing = F) : argument 1 is not a vector
```