Implement Text from Lark

[phrohdoh]

The Lark project, which heavily influences the design of oraide, has a struct named Text that is essentially a slice of a String.

Lark uses this type to avoid expensive clones of Strings in and out of the salsa database which we currently do a lot of.

Implementing and using Text will theoretically improve performance of oraide but we have no numbers to support this claim currently.

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I'm not sure yet in which component to put this Text type (or if a new component should be created).