[ExpressionLanguage] made ExpressionLanguage::parse return an ParsedExpression instance
[ExpressionLanguage] optimized serialization of nodes and expressions
[ExpressionLanguage] renamed addFunction() to register()
merged branch fabpot/expression-language-tweaks (PR #9091)

This PR was merged into the master branch.

Discussion
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[ExpressionLanguage] made minor tweaks

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9087
| License       | MIT
| Doc PR        | n/a

 * made ExpressionLanguage::parse return an ParsedExpression instance
 * optimized the size of the serialization of expressions

Commits
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e869136 [ExpressionLanguage] renamed addFunction() to register()
5076ec7 [ExpressionLanguage] optimized serialization of nodes and expressions
60b9f85 [ExpressionLanguage] made ExpressionLanguage::parse return an ParsedExpression instance