call function for invoking recipes in expressions

[casey]

I haven't really thought very hard about this, but it seems like it should be possible to create a call function which takes as its first argument a recipe name, and then invokes that recipe with some arguments, and returns whatever that recipe writes to standard out:

# baz evaluates to "Hello, bar!"
baz := call(foo, "bar");

foo message:
  echo "Hello, {{ message }}!"

This could also be used to evoke recipes in the middle of other recipes:

bar:
  # {{ call(foo) }}

Without needing to recursively invoke just.

[casey]

I'm sure there are tons of weird considerations and edge cases, like what if a recipe depends on a variable whose value depends on an invocation of call. In that particular case though, this is the same kind of loop detection that we already do for variables and recipe dependencies, and if all calls to call are statically resolvable (which they should be), then we could detect this easily.

[casey]

Also, this function could not be parsed like normal functions, since its argument is not a value, but a recipe, so maybe it needs its own syntax:

(foo)("bar")
foo.call("bar")
…