Core.St function heads more docs-friendly

Having function head like this:
```
def push(state = %{stack: s}, elem)
```
rendered docs don't show the name of the 1st argument:
`push(arg1, elem)`

After switching the sides of the in-head match:
```
def push(%{stack: s} = state, elem)
```
1st arg name gets copied to the doc: `push(state, elem)`