[Lang] Add Boolean operators like `&&`, `||`

[ngernest]

(Summarizing what was discussed in today's meeting, 3/2)

The example in #205 indicates that having Boolean operators is useful to model a larger variety of latency-insensitive interfaces (e.g. Jang et al's Hazard Interfaces).

Remarks:

• This is a relatively straightforward addition to the language, since the well-formedness conditions on guards for if / while ensure that the monitor always has all the information it needs to evaluate the guard in the current clock cycle
• We'll want to disallow && and || from appearing on the RHS of assignments / in assert_eq

NB: this is a nice-to-have extension, but is not essential for the paper submission, so we should de-prioritize for now