Fix various corner cases with operators on Numbers

These only affected particular type combinations, with either
type instabilities which affected inference of return type
or plain failures. Tests are included in the next commit.

base/complex.jl

@@ -461,7 +461,7 @@ function ^{T<:AbstractFloat}(z::Complex{T}, p::Complex{T})
  if p==2 #square
  zr, zi = reim(z)
  x = (zr-zi)*(zr+zi)
- y = 2zr*zi
+ y = T(2)*zr*zi
  if isnan(x)
  if isinf(y)
  x = copysign(zero(T),zr)

base/gmp.jl

@@ -428,6 +428,7 @@ end
 ^(x::BigInt , y::Bool ) = y ? x : one(x)
 ^(x::BigInt , y::Integer) = bigint_pow(x, y)
 ^(x::Integer, y::BigInt ) = bigint_pow(BigInt(x), y)
+^(x::Bool , y::BigInt ) = Base.power_by_squaring(x, y)
 
 function powermod(x::BigInt, p::BigInt, m::BigInt)
  r = BigInt()

base/intfuncs.jl

@@ -186,7 +186,7 @@ ndigits0z(x::Integer) = ndigits0z(unsigned(abs(x)))
 
 const ndigits_max_mul = Core.sizeof(Int) == 4 ? 69000000 : 290000000000000000
 
-function ndigits0znb(n::Int, b::Int)
+function ndigits0znb(n::Signed, b::Int)
  d = 0
  while n != 0
  n = cld(n,b)