Force Julia to specialize on ::Function arguments.

[odow]

See the section in Performance Tips:
https://docs.julialang.org/en/v1/manual/performance-tips/index.html#Be-aware-of-when-Julia-avoids-specializing

This should help simplify (and improve) the precompilation files.

Benchmark

Using the script from jump-dev/JuMP.jl#2484

Before

(base) oscar@Oscars-MBP scripts % '/Applications/Julia-1.6.app/Contents/Resources/julia/bin/julia' --project=. precompile.jl
 13.389848 seconds (43.71 M allocations: 2.494 GiB, 7.98% gc time, 24.12% compilation time)

After

(base) oscar@Oscars-MBP scripts % '/Applications/Julia-1.6.app/Contents/Resources/julia/bin/julia' --project=. precompile.jl
 11.452046 seconds (37.67 M allocations: 2.103 GiB, 8.23% gc time, 26.85% compilation time)

[odow]

With the precompile statements

(base) oscar@Oscars-MBP scripts % '/Applications/Julia-1.6.app/Contents/Resources/julia/bin/julia' --project=. precompile.jl
  8.106619 seconds (22.41 M allocations: 1.234 GiB, 5.04% gc time, 27.18% compilation time)

[odow]

There are two commits here. I can either split them in two PRs, or we can use "Rebase and merge" to add them as separate commits to master.

[blegat]

If ::F, ...) where {F<:Function} is now the default unless we have a good reason to do ::Function, ..) in which case we should add a comment explaining why then we should probably add something saying this in the JuMP style guide

[odow]

So it's not the case that we want this as the default. The heuristic is there for a good reason: most times it is more expensive to compile the method than the added runtime benefit.

I've gone through and removed some of the cases that didn't do anything.

[odow]

(base) oscar@Oscars-MBP scripts % '/Applications/Julia-1.6.app/Contents/Resources/julia/bin/julia' --project=. precompile.jl
 11.481412 seconds (37.60 M allocations: 2.098 GiB, 8.62% gc time, 26.81% compilation time)

[odow]

Rebased onto master which contains the precompile statements:

(base) oscar@Oscars-MBP scripts % '/Applications/Julia-1.6.app/Contents/Resources/julia/bin/julia' --project=. precompile.jl
  8.253007 seconds (23.77 M allocations: 1.312 GiB, 5.16% gc time, 28.03% compilation time)