[embedded] Port Swift.String to embedded Swift

[kubamracek]

This is a port of String, as it is, to Embedded Swift. While a lot of embedded developers might not want Unicode-compliant, dynamically allocated strings in their programs, some might and so let's enable that, and explore "other" strings separately.

Notably, this enables:

• string interpolations
• printing with string interpolations
• using the usual dynamic strings operations, like concatenation, splitting, iteration, etc.

Notably, this does not enable:

• printing of arbitrary types via reflection
• string intepolating arbitrary types via reflection
• String(describing: ...) and String(reflecting: ...) of arbitrary types
• converting arrays, dictionaries into strings with their elements described

This PR roughly contains this:

• Enable building the parts of the stdlib that implement String in the embedded stdlib.
• Add missing refcounting facilities that are needed for String into the embedded runtime, notably bridgeObject refcounting and immortal pointer bit handling.
• Remove a lot of stubs and annotations that were previously disabling strings and string related code.
• Implement integer to string conversions in Swift
• Tweak and ifdef out the usage of AnyObject in StringObject.swift and related code.
• Adopt typed throws in validateUTF8.
• Add a bunch of runtime tests.

[kubamracek]

@mikeash @al45tair I wonder if I could ask for a review of the EmbeddedRuntime.swift changes specifically (and any other part of the PR too if you're interested, of course)

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[phausler]

We still need to follow up on this to introduce a flag for the data tables inclusion as well as a flag to turn off strings entirely (due to code-size impact).

These improvements can be done post-facto imho.

[al45tair]

EmbeddedRuntime changes look good to me.

[MihaelIsaev]

It looks amazing! 🥳
I can't wait to see it next nightly build and test it with Embedded Swift with Webssembly, since we really need String in web development.
How soon it could be available in development snapshot?

[kubamracek]

@swift-ci please build macOS toolchain

[kubamracek]

@swift-ci Please Build Toolchain macOS Platform

[MaxDesiatov]

It looks amazing! 🥳 I can't wait to see it next nightly build and test it with Embedded Swift with Webssembly, since we really need String in web development.

If you're using Embedded Swift to reduce final binary size, using String in your project will negate those gains by bringing Unicode tables back, and generics specialization can even make binaries even bigger in large projects when compared to non-embedded Swift. What you'd need on the web for efficiency is a special string type, ideally that uses UTF-16 under the hood to also avoid UTF-8 <-> UTF-16 re-encoding overhead, since JavaScript strings are UTF-16.

How soon it could be available in development snapshot?

After a PR is merged, development snapshots are tagged nightly on a condition that all tests are passing on all platforms, which may not always be the case. Last time this happened on 1st of May, for a new snapshot to be created again all of the failing toolchain snapshots have to be fixed on all officially supported platforms.

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci Please Build Toolchain macOS Platform

[kubamracek]

@swift-ci please test