Declare tentative return types for ext/spl - part 2

Closes GH-7235

Generate ext/spl class entries from stubs

Closes GH-6709

Add support for `@implementation-alias` in stubs

Closes GH-6170

Add more precise type info for stubs

Closes GH-6005

Include stub hash in generated arginfo files

The hash is used to check whether the arginfo file needs to be
regenerated. PHP-Parser will only be downloaded if this is actually
necess

Include stub hash in generated arginfo files

The hash is used to check whether the arginfo file needs to be
regenerated. PHP-Parser will only be downloaded if this is actually
necessary.

This ensures that release artifacts will never try to regenerate
stubs and thus fetch PHP-Parser, as long as you do not modify any
files.

Closes GH-5739.

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Generate methods entries from stubs for ext/spl

Closes GH-5458

Add stubs for SplHeap and SplPriorityQueue

Closes GH-5266