Automatically reuse pattern variables

[johnynek]

consider a function like this:

def if_one_two_none_zero(o: Option[Int]):
    match o:
        case Some(i) if i matches 1: Some(2)
        case Some(i): Some(i)
        case None: Some(0)

we could have written this as:

def if_one_two_none_zero(o: Option[Int]):
    match o:
        case Some(i) if i matches 1: Some(2)
        case Some(_) as some: some
        case None: Some(0)

but it is longer and a bit noisier, but it would avoid an allocation. It would be nice to optimize the first into the second: namely, any values that are identical to parts of the match, just use the reference to the match instead.

[johnynek]

design doc:

https://johnynek.github.io/bosatsu/design-docs/issue_1628_pattern_value_reuse_design.html