rename IO sub "spew" to "spurt"

[tbrowder]

changes to file "src/core/IO.nqp":

• rename sub "spew" to "spurt" for consistency with
  the rest of Perl

changes to file "t/nqp/019-file-ops.t":

• add tests for sub "spurt"

changes to file "docs/built-ins.md":

• change "spew" to "spurt" in subs
• add "get" to the documented methods on a file handle
• remove full documentation on the slurp method
  on a file handle (but add it the the list
  of available file handle methods)

[tbrowder]

I did check all nqp source and test files, as well as all the Perl 6 and nqp source and test
files in the rakudo repo and roast repo to ensure that “spew” isn’t being used.

[AlexDaniel]

I don't mind this change at all, although I wonder why it was called spew in the first place.

[jnthn]

@AlexDaniel Because it was added before the decision to name this functionality spurt in Perl 6.

👍 on aligning the NQP sub's name with the Perl 6 one.