Prior to this change, every evaluation re-parses the expression. The
intermediate form as a parse tree is never stored anywhere. This change
adds a new token type, FUNCALL, which acts as a node in a parse tree, and
adds an (untested) evaluation method which returns the unevaluated parse tree
as opposed to evaluating as it goes.
This is preparation for a following step in which rather than storing a token
sequence for each variety of content, teapot will store a single token
representing the parse tree, allowing for quicker evaluation when the time
comes; no actual parsing will have to occur on evaluation.
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