Print nested let-abbreviations in ExprSMTLIBPrinter

This patch introduces nested let-abbreviations in the ExprSMTLIBPrinter
to reduce the size of the SMTLIBv2 queries and the corresponding processing
time (bugfix for #170).

The current implementation of the let abbreviation mode does not consider
expression intra-dependencies and prints all abbreviations in the same
let scope. For a (simplified) example, it prints

(assert (let ( (?B1 (A + B)) (?B2 (A + B + C)) ) (= ?B1 ?B2) ) ).

This is extremely inefficient if the expressions (and there many of these!)
extensively reuse their subexpressions. Therefore, it's better to print
the query with nested let-expressions by reusing existing expression bindings
in the new let scope:

(assert (let ( (?B1 (A + B)) ) (let ( (?B2 (?B1 + C)) ) (= ?B1 ?B2) ) ) ).

This patch adds a new function ExprSMTLIBPrinter::scanBindingExprDeps() that
scans bindings for expression dependencies. The result is a vector of
new bindings (orderedBindings) that represents the expression dependency tree.

When printing in the let-abbreviation mode, the new code starts with
abbreviating expressions that have no dependencies and then gradually makes
these new bindings available in the upcoming let-scopes where expressions
with dependencies reuse them.

The effect of nested let-abbreviations is comparable to :named abbreviations.
However, the latter mode is not supported by the majority of the solvers.
Clean up a few comments in ExprSMTLIBPrinter
Clean up some ExprSMTLIBPrinter code by using llvm_unreachable
Add a few line breaks to make the code more readable in
ExprSMTLIBPrinter