StringTemplates deferred evaluation

[Justin Spindler]

I was toying around with the second preview of StringTemplates and I had a
question regarding their design.  I was wondering if it had been considered
for the embedded expressions to be evaluated lazily?

One of the first use cases that came to mind when I was exploring
StringTemplates is logging.  That is an extremely common case where we want
to produce a form of interpolated value, and the current syntax generally
has the same concerns that a formatted string would, in that the inputs are
removed from where they are defined within the message format.  However, if
the log message is below the log level threshold you generally don't want
to incur the cost of building the message, including evaluating those
embedded expressions.  Log libraries typically offer a mechanism to defer
evaluation via Suppliers, but that feels like it would be a challenge to
use within StringTemplate.

C# is an example of a language that offers this functionality via
FormattableString, which gives a method the ability to choose whether or
not to interpret the template or evaluate the expressions.  That allows
logging below threshold to be more or less a no-op.
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