Move nsstring from gecko into servo/support/gecko/nsstring

[mystor]

This is the servo side of bug 1403213.

Take 2 of #18941 which got backed out because we botched the landing of the gecko side and I wasn't able to fix it in time.

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @bholley: components/style/Cargo.toml, components/style/lib.rs, ports/geckolib/Cargo.toml, components/style/gecko_bindings/nsstring_vendor/Cargo.toml, components/style/gecko_bindings/nsstring_vendor/src/lib.rs
• @canaltinova: components/style/Cargo.toml, components/style/lib.rs, components/style/gecko_bindings/nsstring_vendor/Cargo.toml, components/style/gecko_bindings/nsstring_vendor/src/lib.rs
• @edunham: servo-tidy.toml
• @emilio: components/style/Cargo.toml, components/style/lib.rs, ports/geckolib/Cargo.toml, components/style/gecko_bindings/nsstring_vendor/Cargo.toml, components/style/gecko_bindings/nsstring_vendor/src/lib.rs

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!
• These commits modify style code, but no tests are modified. Please consider adding a test!

[mystor]

r? @Manishearth
I'm going to push the patch to mozreview on the gecko side so we make sure we land the right thing this time :-).

[Manishearth]

@bors-servo delegate+ r+ p=10

[bors-servo]

📌 Commit 2d3331f has been approved by Manishearth

[bors-servo]

✌️ @mystor can now approve this pull request

[mystor]

@bors-servo r+

I have the gecko side commits ready to land on reviewboard, so I think this'll go dandy this time :-).

[bors-servo]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: #18994

[bors-servo]

📌 Commit 2d3331f has been approved by mystor

[bors-servo]

⌛ Testing commit 2d3331f with merge 0e6946f...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css1, mac-rel-css2, mac-rel-wpt1, mac-rel-wpt2, mac-rel-wpt3, mac-rel-wpt4, windows-msvc-dev
Approved by: mystor
Pushing 0e6946f to master...