Support `f32`/`f64` in native function calls.

[weiznich]

Currently this results in an error like that one:

error: unsupported operation: unsupported return type for native call: "sqlite3_value_double"
    --> /home/weiznich/Documents/rust/diesel/diesel/src/sqlite/connection/sqlite_value.rs:186:18
     |
 186 |         unsafe { ffi::sqlite3_value_double(self.value.as_ptr()) }
     |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsupported operation occurred here
     |
     = help: this is likely not a bug in the program; it indicates that the program performed an operation that Miri does not support
     = note: BACKTRACE on thread `types::f32_from`:
     = note: inside `diesel::sqlite::SqliteValue::<'_, '_, '_>::read_double` at /home/weiznich/Documents/rust/diesel/diesel/src/sqlite/connection/sqlite_value.rs:186:18: 186:64

It would be nice to support floating point types there as well.

[RalfJung]

Marking as good-first-issue since this should be fairly easy:

• add f32 and f64 in this match (for float arguments):

  miri/src/shims/native_lib/mod.rs

  Line 407 in ce6f40d

  return interp_ok(match s.primitive() {

• and also add them in this match (for float return values):

  miri/src/shims/native_lib/mod.rs

  Line 95 in ce6f40d

  let scalar = match dest.layout.ty.kind() {

• ensure we have tests for floats in both argument and return position