Global type inference (function signatures without types)

[Centril]

Either for development purposes, or in general, we could add global type inference to Sway. For example, you could define a function:

fn foo(a, b) { a * b }

and a type signature could be inferred for foo.

Then we can decide either to emit the inferred type as an error, as a design decision. Or, we could allow the function foo as legal code.

Another related concept is introducing typed holes, like so:

fn foo(a: ?, b: ?) { a * b }

[Centril]

cc @emilyaherbert