Allow placeholders to be optional in multiplication operations

[k4b7]

rule: #a_0?1 #b + ... --> (#a_0 + ...) #b
example: x + 2x + 3x --> (1 + 2 + 3) x

The thinking is that ? would indicate that the placeholder before it is optional. The value after the ? would be the value to use if the placeholder doesn't exist. We could also not bother with this fallback value b/c for multiplication we'll almost always want this to be 1.