Use ByteStrings for strings that don't use FastString's features

The first two candidates that I have in mind are SourceText and StringLiteral's sl_fs field.

There might be a point in creating a simple wrapper for UTF8-encoded ByteString. HsDocString could use that one too.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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CC	alexbiehl
Operating system
Architecture

changed milestone to %8.8.1

changed weight to 5

added Ttask Trac import strings labels

Note that we also have LitString for literal strings. Otherwise I agree that a newtype-wrapped UTF-8 ByteString would make sense here.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

mentioned in issue #16190 (closed)

added 1 deleted label

added compiler perf label and removed 1 deleted label

added Pnormal label

Somewhat related to #17259

removed milestone

added faststring label