sys_core_fold: Optimize let statements more aggressively

I originally decided that in 'value' context, rewriting a let statement
where the variables were not in the body to a sequence was not worth
it, because the variables would be unused in only one let in a
thousand lets (roughly).

I have reconsidered.

The main reason is that if we do the rewrite, core_lib:is_var_used/2
will be used much more frequently, which will help us to find bugs
in it sooner.

Another reason is that the way letify/2 is currently implemented
with its own calls to core_lib:is_var_used/2 is only safe as long
as all the bindings are independent of each other. We could make
letify/2 smarter, but if we introduce this new optimization there
is no need.

Measuring compilation speed, I have not seen any significant slowdown.
It seems that although core_lib:is_var_used/2 is called much more
frequently, most calls will be fast because is_var_used/2 will quickly
find a use of the variable.

Also add a test case to cover a line opt_guard_try/1 that was
no longer covered.

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