Add .unaryValue to ExecutionValue

[benjie]

Description

ExecutionValue comes in two flavours: batch and unary. Unary has a .value and batch has a .entries. Previously we included : never versions of each of these to make access easier, but this could lead to mistakes. This interface has been removed (:rotating_light: breaking! but only the types) so you'll need to narrow via .isBatch before you can use the relevant property.

When you knew an execution value was unary, you could use .at(0) to access its value; however this was risky because there was no assertion that the value is unary. To help mitigate this, I've added the new .unaryValue() method which asserts that the value is unary and then accesses the singular value from it; this should make code safer when you know a $step should be unary due to calling this.addUnaryDependency($step).

Performance impact

Negligible.

Security impact

Improvement: increased safety. Risk: assertions.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.