Merge pull request #4781 from fabianlischka/patch-1

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Fix logic error in process_exited(s::ProcessChain)
initial revision
bitarray: new reshape/reinterpret messages (see #4765)
Merge pull request #4791 from JuliaLang/cjh/process_exited_logic

Fix logic error in process_exited(s::ProcessChain)
Second attempt to fix logic error in process status functions

Fix #4791.
remove _MISSING value in dict. closes #4802
document scope of keyword and optional argument default values. closes #4795
fix #4801, keyword arg in constructor on first call
fix some of the issues in #4807, arrays of Complex{T<:Real}
make gcdx type-stable & 20x faster, fix invmod to work with negative arguments, use GMP for BigInt invmod
improve matmul result type. closes #4807, closes #4796
make gcd and lcm positive (fix #4810)
RFC: Fixes issue 3453, string literal input formats for Int128 & Uint128
update gcd/lcm/gcdx docs
Remove quoting from @int128_str & @uint128_str
ensure consistent results for gcdx(0,0) with BigInt
bugfix for lcm(0,0), add test coverage of gcd-related functions
fix performance bug in assert

also fix round-trip value<->expression conversion of `nothing`

in codegen for `quote`, avoid generating jl_copy_ast calls for atoms
domain error for negative powers in BigInt powermod