Eta expand functions with optional parameters when resolving implicits

[TimWhiting]

When using a function with an optional parameter as implicit, the types do not unify. Eta-expanding the function fixes the issue. This is particularly important due to double/show having the default parameter for precision.

[daanx]

Hi Tim -- thanks for the PR, I think it looks good. However, I have generalized this idea a bit and added code to automatically eta-expand any variable argument that has optional parameters but the expected type does not. For example, in:

fun plus(x:int,y:int,z:int=1)
  x+y+z

pub fun main()
  [1,2,3].foldl(0,plus)

I think the commit 123d3e7 also handles what this PR is doing -- but I'm not 100% sure; maybe you can see if it covers your use cases?

[TimWhiting]

@daanx
This still fails: stack run koka -- -e test/syntax/run/tail. Also apparently the additional config.json options in the run directory are not replacing or being added the options in the parent directory, so the run tests aren't actually being run.

[daanx]

Ah, I think this is a different problem -- namely, the main that is generated does a println on a float64 but it doesn't import std/num/float64. This is something we should fix as well.

[daanx]

I think the import problem is fixed now in the latest dev (commit f05b93c) . (the tail tests work)