Switch to 1.31.0, not 1.30.0
Set correct toolchain in CI

This is one step closer toward 1.32.0, where the 2018 edition is stable.
Mostly refactoring the imports and giving function parameters name in traits

Refactor the assignements to reduce complexity

See merge request !901

Reorder the branches to extract the common patterns
Use Results to move error handling upper in the chain.
Use as much internal functions, such as `map` to reduce match nesting.
Use iterators when applicable.

Bit of refactoring for variables and assignment

See merge request !900

Clippy and fmt

See merge request !897

Use iterators and a helper function to implement Terminator in a clearer way (high cyclomatic complexity in previous implementation). Use an enum to detect error types to deal with them in another scope without requiring an artificial block, as well as to avoid the overhead of dealing with bitflags when not needed.
Use native iterators whenever possible, and create a custom one to keep track of the last two characters found.

Add toolchain in Makefile to remove the need to set a system-wide target toolchain but run the test against the right version

Add consistent prefix in Compile Instructions

See merge request !894

Fix termination of quoted strings containing "<<"

Closes #842

See merge request !895

Add the ability to use numbers for indexing map types

Closes #840

See merge request redox-os/ion!891

Allow for empty arrays when printing all variables

Closes #841

See merge request redox-os/ion!893

Update debian packaging

See merge request redox-os/ion!892

Make map value reassignment actually work

Closes #839

See merge request redox-os/ion!890

Merge branch 'xTibor/doc-btreemap'

Add the ability to get the last exit status from any scope

Closes #812

See merge request !888