This commit adds support for lambda expressions to Poke. Lambda
expressions use the following simple syntax:
lambda FUNCTION_SPECIFIER
Where a FUNCTION_SPECIFIER is the same notation that one would use
when defining a function in a `fun' construction. Examples:
(poke) lambda void: {}
#<closure>
(poke) lambda (int i) int: { return i + 2; }
#<closure>
Lambdas can be invoked like any other function value:
(poke) lambda void: {} ()
(poke) lambda (int i) int: { return i + 2; } (10)
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Lambdas can also be stored in variables:
(poke) var la = lambda (int i) int: { return i + 2; }
(poke) la (10)
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