Zejun Wu authored Oct 28, 2018 and Ben Gamari committed Oct 28, 2018

Summary:
Reimplement global FastString table using a concurrent hashatable with
fixed size segments and dynamically growing buckets instead of fixed size
buckets.

This addresses the problem that `mkFastString` was not linear when the
total number of entries was large.

Test Plan:
./validate

```
inplace/bin/ghc-stage2 --interactive -dfaststring-stats < /dev/null
GHCi, version 8.7.20181005: http://www.haskell.org/ghc/  :? for help
Prelude> Leaving GHCi.
FastString stats:
    segments:          256
    buckets:           16384
    entries:           7117
    largest segment:   64
    smallest segment:  64
    longest bucket:    5
    has z-encoding:    0%
```

Also comapre the two implementation using

{P187}

The new implementation is on a par with the old version with different
conbination of parameters and perform better when the number of
FastString's are large.

{P188}

Reviewers: simonmar, bgamari, niteria

Reviewed By: simonmar, bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #14854

Differential Revision: https://phabricator.haskell.org/D5211