Rewrite decodeURI and decodeURIComponent as builtin functions
and install them in the bootstrapper.

Delete unused runtime functions:
 - TruncateString
 - NewString
 - OneByteSeqStringGetChar
 - OneByteSeqStringSetChar
 - TwoByteSeqStringGetChar
 - TwoByteSeqStringSetChar

Add regression test for decoding large strings. Clusterfuzz detected
a problem with %TruncateString, see
https://bugs.chromium.org/p/chromium/issues/detail?id=612109#c6
This is automatically fixed by this rewrite because %TruncateString
is deleted anyways.

Crude benchmark on 585 decodeURI and decodeURIComponent tests
averaged over five runs:

* builtin functions
real	0m9.69s
user	2m39.8816s
sys    	0m12.6398s

* JS functions calling into the runtime e.g., for %TruncateString
real	0m11.0598s
user	3m6.7026s
sys	0m13.5756s

By running:
$  time tools/run-tests.py   --arch=x64  --mode=Release --buildbot
  test262/built-ins/decodeURI* mjsunit/uri
>>> Running tests for x64.Release

BUG=v8:4912, chromium:612109
R=yangguo@chromium.org, bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/1994733003
Cr-Commit-Position: refs/heads/master@{#36543}