impl `#[map]`

[ariasanovsky]

#[spindle::map]
fn foo(x: X) -> Y { /* blah */ }

should serialize to

$PROJECT/
└── target/
    └── spindle/
        └── map/
            └── foo.json

so that

spindle::spin!(U, X, Y)

can find it.

[ariasanovsky]

Resolved!