[meta] Expression, Primary expression

[Hywan]

Address #14.

Specification

https://github.com/php/php-langspec/blob/master/spec/19-grammar.md#expressions

Progression

• variable (must be reviewed, it has changed),
• class-constant-access-expression,
• constant-access-expression,
• literal,
• array-creation-expression,
• intrinsic,
• anonymous-function-creation-expression,
• ( expression ).

[Hywan]

variable definition has changed, https://github.com/php/php-langspec/blob/master/spec/10-expressions.md#grammar-variable. Our implementation must be updated.