Interpolation should allow inserting Expression-Words without pre- and appended spaces.

[bj2015]

Currently, Interpolation in KernelF automatically pre- and appends spaces whenever inserting Expression-Words:

val e = "bar" '''foo$(e)baz'''

yields "foo bar baz".

In order to give the user more control over the spacing, there should be a special character "~", which allows omitting these inserted spaces, i.e. "foo~$(e)~baz" would yield "foobarbaz" and "foo~$(e)baz" would yield "foobar baz".