Rename `move(along:)` to `move(by:)` based on the proposal feedback. The main argument for the change is that tangent vectors specify both a direction and a magnitude, whereas `along:` does not indicate that `self` is being moved by the specified magnitude.
Add test/AutoDiff/lit.local.cfg: run tests only when `differentiable_programming`
is enabled in lit. With this, individual tests no longer need
`REQUIRES: differentiable_programming`.
Move multi-functionality SIL tests from test/AutoDiff/SIL/Serialization to
test/AutoDiff/SIL.
Garden test filenames.
Add `Differentiable` conformances for floating-point types to the
`_Differentiation` module. The `TangentVector` associated type for
floating-point types is `Self`.
This design adheres to the differentiable programming manifesto:
docs/DifferentiableProgramming.md.
Partially resolves TF-1052.
