use pairwise summation for sum, cumsum, and cumprod
use pairwise summation for mean, var, varm; also fix bug in var for complex arrays (should use absolute value and return a real number)
use pairwise reduction for sum(f, A) and prod(f, A) via new (non-exported) mapreduce_associative
clean up some reflection functions

methods(f, t) returns an array of Methods
code_lowered returns an array of LambdaStaticData (fixes #4023)
code_typed returns an array of ASTs
_methods returns tuples (type, env, Method)
code_llvm, code_native: error if no method found
allow code_llvm, code_native to work on constructors
add a fallback that can convert any Integer to BigInt, 32 bits at a time
should fix #4043
Merge pull request #4039 from stevengj/pairwise

RFC: use pairwise summation for sum, cumsum, and cumprod