Avoid using element equality for finding elements in circuit

Note the following change:

Before:
```julia
julia> R = resistor(1000); circ = Circuit();

julia> add!(circ, R)
Symbol("##296")

julia> add!(circ, R)
Symbol("##296")

julia> ACME.nb(circ)
1
```
After:
```julia
julia> R = resistor(1000); circ = Circuit();

julia> add!(circ, R)
Symbol("##296")

julia> add!(circ, R)
Symbol("##297")

julia> ACME.nb(circ)
2
```
I.e. the same element can now be added twice. In general, element
instances should not have an identity, so the new behaviour seems more
reasonable. (The same `R` could already be added to another circuit
without having any effect on its presence in the first one, for
example.)

src/circuit.jl

@@ -1,4 +1,4 @@
-# Copyright 2015, 2016, 2017, 2018, 2019, 2020, 2021 Martin Holters
+# Copyright 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2023 Martin Holters
 # See accompanying license file.
 
 export Circuit, add!, connect!, disconnect!, @circuit, composite_element
@@ -53,9 +53,8 @@ function incidence(c::Circuit)
     j = sizehint!(Int[], 2nb(c))
     v = sizehint!(Int[], 2nb(c))
     for (row, pins) in enumerate(c.nets), (elemname, pinname) in pins
-        elem = c.elements[elemname]
-        offset = branch_offset(c, elem)
-        bps = elem.pins[pinname]
+        offset = branch_offset(c, elemname)
+        bps = c.elements[elemname].pins[pinname]
         for (branch, polarity) in bps
             push!(i, row)
             push!(j, offset + branch)
@@ -93,11 +92,6 @@ Adds the element `elem` to the circuit `c`, creating and returning a new, unique
 reference designator, leaving its pins unconnected.
 """
 function add!(c::Circuit, elem::Element)
-    for (k, v) in c.elements
-        if v == elem
-            return k
-        end
-    end
     designator = gensym()
     add!(c, designator, elem)
     return designator
@@ -135,10 +129,10 @@ function Base.delete!(c::Circuit, designator::Symbol)
     delete!(c.elements, designator)
 end
 
-function branch_offset(c::Circuit, elem::Element)
+function branch_offset(c::Circuit, designator::Symbol)
     offset = 0
-    for el in elements(c)
-        el == elem && return offset
+    for (des, el) in c.elements
+        des === designator && return offset
         offset += nb(el)
     end
     throw(ArgumentError("Element not found in circuit"))