Term printing is not unicode compatible

[MasonProtter]

Currently, the printing of Terms assumes that the string representation of the term may be sanely indexed which is not actually the case whenever there is unicode in the Term.

MWE:

julia> using Terms

julia> @term :ω
Error showing value of type Term:
ERROR: StringIndexError(":(@term :ω)", 11)
Stacktrace:
 [1] string_index_err(::String, ::Int64) at ./strings/string.jl:12
 [2] getindex at ./strings/string.jl:248 [inlined]
 [3] show(::IOContext{REPL.Terminals.TTYTerminal}, ::Term) at /Users/mason/.julia/packages/Terms/rb9Kk/src/term.jl:58
 [4] show(::IOContext{REPL.Terminals.TTYTerminal}, ::MIME{Symbol("text/plain")}, ::Term) at ./sysimg.jl:194
 [5] display(::REPL.REPLDisplay, ::MIME{Symbol("text/plain")}, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:131
 [6] display(::REPL.REPLDisplay, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:135
 [7] display(::Any) at ./multimedia.jl:287
 [8] #invokelatest#1 at ./essentials.jl:742 [inlined]
 [9] invokelatest at ./essentials.jl:741 [inlined]
 [10] print_response(::IO, ::Any, ::Any, ::Bool, ::Bool, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:155
 [11] print_response(::REPL.AbstractREPL, ::Any, ::Any, ::Bool, ::Bool) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:140
 [12] (::getfield(REPL, Symbol("#do_respond#38")){Bool,getfield(REPL, Symbol("##48#57")){REPL.LineEditREPL,REPL.REPLHistoryProvider},REPL.LineEditREPL,REPL.LineEdit.Prompt})(::Any, ::Any, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:714
 [13] #invokelatest#1 at ./essentials.jl:742 [inlined]
 [14] invokelatest at ./essentials.jl:741 [inlined]
 [15] run_interface(::REPL.Terminals.TextTerminal, ::REPL.LineEdit.ModalInterface, ::REPL.LineEdit.MIState) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/LineEdit.jl:2273
 [16] run_frontend(::REPL.LineEditREPL, ::REPL.REPLBackendRef) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:1035
 [17] run_repl(::REPL.AbstractREPL, ::Any) at /Users/mason/julia/usr/share/julia/stdlib/v1.1/REPL/src/REPL.jl:192
 [18] (::getfield(Base, Symbol("##734#736")){Bool,Bool,Bool,Bool})(::Module) at ./client.jl:362
 [19] #invokelatest#1 at ./essentials.jl:742 [inlined]
 [20] invokelatest at ./essentials.jl:741 [inlined]
 [21] run_main_repl(::Bool, ::Bool, ::Bool, ::Bool, ::Bool) at ./client.jl:346
 [22] exec_options(::Base.JLOptions) at ./client.jl:284
 [23] _start() at ./client.jl:436

Here is some stuff to show some intuition for the problem:

str = "a ω b"

julia> length(str)
5

julia> str[1]
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)

julia> str[2]
' ': ASCII/Unicode U+0020 (category Zs: Separator, space)

julia> str[3]
'ω': Unicode U+03c9 (category Ll: Letter, lowercase)

julia> str[4]
ERROR: StringIndexError("a ω b", 4)
Stacktrace:
 [1] string_index_err(::String, ::Int64) at ./strings/string.jl:12
 [2] getindex_continued(::String, ::Int64, ::UInt32) at ./strings/string.jl:218
 [3] getindex(::String, ::Int64) at ./strings/string.jl:211
 [4] top-level scope at none:0

julia> eachindex(str)
Base.EachStringIndex{String}("a ω b")

julia> for i in eachindex(str)
       println(str[i])
       end
a

ω

b