Name intermediate Leaf types

[Eyas]

Rather than inlining a "leaf" type as an intersection in a big union,
name it. This should hopefully help with #34.

This is based on advice from @amcasey regarding compiler performance:

I've definitely seen some impressive perf improvements from naming
intermediate types (because it can short-circuit structural type
comparison, which is very expensive).

This fits neatly with my initial description of the Schema.org type
system here:
https://blog.eyas.sh/2019/05/modeling-schema-org-schema-with-typescript-the-power-and-limitations-of-the-typescript-type-system/

There's still some room for improvements:

• Some intermediate types are simply aliases, can we remove these?
• 'DataType's don't necessarily fit neatly here.

Also in this PR,

• Remove unnecessary parens from certain type unions.

[amcasey]

@Eyas Cool! What sort of perf gain did you see?

For the benefit of future readers, I should clarify that when naming intermediate types helps (which is rare), it can help a lot.

[Eyas]

It makes a bit of a (positive) difference in 3.7 and 3.8, but might have regressed in 3.9.1. 95% confidence interval is pretty wide though.

TS 3.7:

• 0.5.2: Mean memory of 5 runs: 95179.2K (95 CI: ±3,443)
• 0.5.3: Mean memory of 5 runs: 92897.2K (95 CI: ±2,697)

TS 3.8:

• 0.5.2: Mean memory of 5 runs: 100053K (95 CI: ±1,004)
• 0.5.3: Mean memory of 5 runs: 94875K (95 CI: ±2,677)

TS 3.9.1-rc:

• 0.5.2: Mean memory of 5 runs: 96871.6K (95 CI: ±231)
• 0.5.3: Mean memory of 5 runs: 99921K (95 CI: ±2,271)

[Eyas]

This is using --extendedDiagnostics and looking at "Memory used". The repro case is simple:

import { WithContext, Article } from "schema-dts";

export const article: WithContext<Article> = {
    "@context": "https://schema.org",
    "@type": "Article",
    name: [
        "Woo", "Hoo"
    ]
};

with either schema-dts version installed. Node 13.

[amcasey]

Thanks! One day, I'll figure out why node memory usage is so variable. 😉