Add libgmp for faster Integer calculations.

[lsb]

I was wondering why some large integer multiplications were taking a while, so I ran

$ docker run ruby:2.6 ruby -e 'puts Integer::GMP_VERSION'
-e:1:in `<main>': uninitialized constant Integer::GMP_VERSION (NameError)

and found that GMP wasn't included in the default images. Building these images gives me

$ docker run ruby:gmp-2.6 ruby -e 'puts Integer::GMP_VERSION'
GMP 6.1.2

For significantly large integers (on my machine) GMP is over 50x faster than Ruby's fallback Integer methods: https://github.com/ruby/ruby/blob/a2eb04f52dccb9b42404f8b133acb36b577e3b54/bignum.c#L2527

[tianon]

Nice catch; I dug in and found ruby/ruby@83a0709, which led me to wonder if this was actually applicable enough to add to buildpack-deps which led me to find:

• https://www.npmjs.com/package/gmplib
• https://pypi.org/project/gmpy/ + https://pypi.org/project/gmpy2/
• https://github.com/ncw/gmp

... and thus file docker-library/buildpack-deps#92. 👍

[tianon]

(but this is still applicable for -slim and -alpine variants, so I'll be pushing an update shortly after I finish a little testing to make sure it works on each)

[maxbrunet]

@juusujanar I got the issue with nokogiri on alpine and fixed it by installing gmp-dev.

[ghost]

jaro_winkler can't build on ruby:2.4.5-slim-stretch. Add libgmp-dev fixed it.

[springerigor]

Since this PR was merged libgmp-dev is removed by apt-get purge command. As a result it is not available during runtime and some Ruby gems depends on it (like well-known nokogiri).

To solve the issue you should install it by yourself (apt get libgmp-dev).

I tested it with Ruby 2.5.3 & Nokogiri 1.8.4 and the described approach solved the issue.

[tianon]

See #267 for where this issue is being discussed further.