Rename AnyPayloadProvider to FixedProvider, make it generic

[Manishearth]

Part 1 of #3947

not yet planning on doing part 2

[robertbastian]

Assuming this is a clean rename

[Manishearth]

Assuming this is a clean rename

No, it changes semantics too, see the issue. It makes it generic so that it no longer relies on storing an AnyPayload

[sffc]

failures:

---- tools/md-tests/src/lib.rs - tutorials::data_provider_md (line 210) stdout ----
error[E0107]: associated function takes 0 generic arguments but 1 generic argument was supplied
  --> tools/md-tests/src/lib.rs:244:20
   |
36 |     FixedProvider::new_default::<DecimalSymbolsV1Marker>()
   |                    ^^^^^^^^^^^-------------------------- help: remove these generics
   |                    |
   |                    expected 0 generic arguments
   |
note: associated function defined here, with 0 generic parameters
  --> /home/runner/work/icu4x/icu4x/provider/adapters/src/fixed.rs:79:12
   |
79 |     pub fn new_default() -> Self
   |            ^^^^^^^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0107`.
Couldn't compile the test.

failures:
    tools/md-tests/src/lib.rs - tutorials::data_provider_md (line 210)

test result: FAILED. 20 passed; 1 failed; 1 ignored; 0 measured; 0 filtered out; finished in 5.89s

error: doctest failed, to rerun pass `-p md-tests --doc`