@set expression parsing issue with Unicode operators in compound assignment

[DaniGlez]

Hi, I think this is supposed to work but errors out instead on 0.1.39

julia> using Accessors

julia> foo(x) = (@set x[1] ⊻= x[2])
ERROR: LoadError: StringIndexError: invalid index [3], valid nearby indices [1]=>'⊻', [4]=>'='
Stacktrace:
 [1] string_index_err(s::String, i::Int64)
   @ Base ./strings/string.jl:12
 [2] getindex
   @ ./strings/string.jl:502 [inlined]
 [3] get_update_op(sym::Symbol)
   @ Accessors ~/.julia/packages/Accessors/p8I9Z/src/sugar.jl:283
 [4] setmacro(optictransform::Function, ex::Expr; overwrite::Bool)
   @ Accessors ~/.julia/packages/Accessors/p8I9Z/src/sugar.jl:318
 [5] var"@set"(__source__::LineNumberNode, __module__::Module, ex::Any)
   @ Accessors ~/.julia/packages/Accessors/p8I9Z/src/sugar.jl:33
in expression starting at REPL[37]:1

whereas these seem to work flawlessly:

julia> foo(x) = (@set x[1] += x[2])
foo (generic function with 1 method)

julia> foo(x) = (@set x[1] = x[1] ⊻ x[2])
foo (generic function with 1 method)

[aplavin]

Nice catch! This line

Accessors.jl/src/sugar.jl

Line 283 in 11e9dc4

Symbol(s[1:end-1])

should probably use chop() instead of end-1 indexing.