Make min/max prefix, and add infix operators like `↓` and `↑`

[byorgey]

Infix max, min are weird and nonstandard.

[byorgey]

While we're at it, could max and min just be standard library functions?

[byorgey]

To define min and max as standard library functions depends on #179. For example:

min : a * a -> a [comparable a]
min(x,y) = {? x if x <= y, y otherwise ?}

Once we have #136 we could also define infix synonyms. I'm actually partial to Dijkstra's notation and ↓ and ↑ for min and max, respectively.

[byorgey]

We shouldn't wait for #179; we should go ahead and make min and max prefix.

• A first pass would be to just remove them from Syntax.Operators and add them as prims.
• Or, instead of removing them, we could change the syntax in Syntax.Operators to ↓ and ↑. The tricky thing is that we need some ASCII syntax for them as well...