experiment: just use function composition for multi migrations

[crusso]

Simpler alternative to #5635 using a simple composition operator (function composition) to compose migrations functions into a single migration.

Currently requires relaxing the restriction that the migration expression is static (i.e. just a value).

Pros:

• simple

Cons:

• doesn't implement the full checks of feat: Improve context dot instantiation used for error reporting, solve only receiver type variables #5634, however, we could probably emulate those by building in a special composition operator and just using that instead of the simple library function Func.then (f.then(g)).