docs: add demos for issue 80 utility methods

[landeqiming666]

Summary

This adds QED-style demos for several utility methods discussed in #80 that already exist in the codebase:

• Enumerable#squeeze
• Enumerator::Lazy#squeeze
• Hash#fetch_nested
• String#rotate / String#rotate!

The issue discussion asked for methods to be submitted with documentation, Lemon tests, and QED demos. These methods already have implementations and related test coverage, so this PR fills in the missing demo/docs side for the existing APIs.

Refs #80

Validation

• git diff --check
• ruby -Ilib/core -e "require 'facets/enumerable/squeeze'; raise unless [1,2,2,3,3,2,1].squeeze == [1,2,3,2,1]; raise unless [1,2,2,3,3,2,1].squeeze(3) == [1,2,2,3,2,1]"
• ruby -Ilib/core -e "require 'facets/enumerator/lazy/squeeze'; raise unless [1,2,2,3,3,2,1].lazy.squeeze.to_a == [1,2,3,2,1]; raise unless [1,2,2,3,3,2,1].lazy.squeeze(3).to_a == [1,2,2,3,2,1]"
• ruby -Ilib/core -e "require 'facets/string/rotate'; raise unless 'abcdefgh'.rotate(2) == 'cdefghab'; raise unless 'abcdefgh'.rotate(-2) == 'ghabcdef'; s='abcdefgh'; s.rotate!(2); raise unless s == 'cdefghab'"
• ruby -Ilib/core -e "require 'facets/hash/fetch_nested'; data={'hello'=>{'world'=>42}}; raise unless data.fetch_nested('hello','world') == 42; raise unless data.fetch_nested('hello','missing').nil?; raise unless data.fetch_nested('hello','missing'){|*keys| keys.join('.')} == 'hello.missing'"

Note: I attempted to install the QED test dependency via Bundler, but dependency resolution did not complete in a reasonable time on this local Ruby 2.6.10 environment.