exact `BigFloat` to IEEE FP conversion in pure Julia (#50691)
There's lots of code, but most of it seems like it will be useful in
general. For example, I think I'll use the changes in float.jl and
rounding.jl to improve the #49749 PR. The changes in float.jl could also
be used to refactor float.jl to remove many magic constants.
Benchmarking script:
```julia
using BenchmarkTools
f(::Type{T} = BigFloat, n::Int = 2000) where {T} = rand(T, n)
g!(u, v) = map!(eltype(u), u, v)
@btime g!(u, v) setup=(u = f(Float16); v = f();)
@btime g!(u, v) setup=(u = f(Float32); v = f();)
@btime g!(u, v) setup=(u = f(Float64); v = f();)
```
On master (dc064683087d017927381e):
```
46.116 μs (0 allocations: 0 bytes)
38.842 μs (0 allocations: 0 bytes)
37.039 μs (0 allocations: 0 bytes)
```
With both this commit and #50674 applied:
```
42.310 μs (0 allocations: 0 bytes)
42.661 μs (0 allocations: 0 bytes)
41.608 μs (0 allocations: 0 bytes)
```
So, with this benchmark at least, on an AMD Zen 2 laptop, conversion to
`Float16` is faster, but there's a slowdown for `Float32` and `Float64`.
Fixes #50642 (exact conversion to `Float16`)
Co-authored-by: Oscar Smith <oscardssmith@gmail.com>
