A few minor perforamnce fixes (#470)

* improve `to_uint64`

Reduces some allocations and intermediate work.

PR:
```
julia> @benchmark JLD2.to_uint64(rand(UInt8, 5))
BenchmarkTools.Trial: 10000 samples with 983 evaluations.
 Range (min … max):  58.730 ns …   1.927 μs  ┊ GC (min … max):  0.00% … 93.54%
 Time  (median):     65.484 ns               ┊ GC (median):     0.00%
 Time  (mean ± σ):   75.603 ns ± 116.148 ns  ┊ GC (mean ± σ):  10.53% ±  6.58%

     ▃▁▆█▆▂
  ▂▄▆███████▆▅▄▄▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▁▂▂▂▂▂▂▂▂▂▂▂▂▂ ▃
  58.7 ns         Histogram: frequency by time          117 ns <

 Memory estimate: 224 bytes, allocs estimate: 4.
```

Master:
```
julia> @benchmark JLD2.to_uint64(rand(UInt8, 5))
BenchmarkTools.Trial: 10000 samples with 981 evaluations.
 Range (min … max):  63.088 ns …  1.688 μs  ┊ GC (min … max): 0.00% … 94.44%
 Time  (median):     69.438 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   78.976 ns ± 92.630 ns  ┊ GC (mean ± σ):  8.47% ±  6.84%

  ▂▄▅▇▇█▇▆▅▅▄▃▃▃▂▂▃▂▂▁▁                                       ▂
  ██████████████████████▇▇▆▆▇▆██▇█▇▇▇██▇▆▆▆▄▁▃▃▃▆▅▆▆▅▆▄▄▃▄▆██ █
  63.1 ns      Histogram: log(frequency) by time       130 ns <

 Memory estimate: 256 bytes, allocs estimate: 4.
```

* fix failure to const-prop in error message

this was not getting const propped, so
just mark as such

* improve type stability in show_group

src/compression.jl

@@ -65,18 +65,17 @@ const ID_TO_DECOMPRESSOR = Dict(
  UInt16(32004) => (:CodecLz4, :LZ4FrameCompressor, :LZ4FrameDecompressor, "LZ4"),
 )
 
+const compressor_list_string = map(values(ID_TO_DECOMPRESSOR)) do val
+ "\n\t"*string(val[1])*"."*string(val[2])
+end
+
 issupported_filter(filter_id) = filter_id ∈ keys(ID_TO_DECOMPRESSOR)
 
 function verify_compressor(compressor)
  (compressor isa Bool || haskey(COMPRESSOR_TO_ID, nameof(typeof(compressor)))) && return
 
- crs = map(values(ID_TO_DECOMPRESSOR)) do val
- "\n\t"*string(val[1])*"."*string(val[2])
- end
-
-
  throw(ArgumentError("""Unsupported Compressor
- Supported Compressors are $(crs...)"""))
+ Supported Compressors are $(compressor_list_string...)"""))
 end
 #############################################################################################################
 # Dynamic Package Loading Logic copied from FileIO

src/groups.jl

@@ -644,15 +644,15 @@ function show_group(io::IO, g::Group, maxnumlines::Int=10, prefix::String=" ", s
  iszero(maxnumlines) && return 0
  if g.f.n_times_opened == 0
  print(io, " (closed)")
- return
+ return -1
  end
 
  ks = collect(keys(g))
  skiptypes && filter!(x -> x != "_types", ks)
 
  if isempty(ks) && prefix == " "
  print(io, " (no datasets)")
- return
+ return -1
  end
 
  for i = 1:length(ks)
@@ -670,7 +670,11 @@ function show_group(io::IO, g::Group, maxnumlines::Int=10, prefix::String=" ", s
  if !isempty(newg)
  if (maxnumlines > 1 || (islast && maxnumlines >= 2))
  print(io, '\n')
- maxnumlines = show_group(io, newg, islast ? maxnumlines : maxnumlines-1, prefix*(islast ? " " : "│ "), false)
+ ret = show_group(io, newg, islast ? maxnumlines : maxnumlines-1, prefix*(islast ? " " : "│ "), false)
+ if ret < 0
+ error("closed or empty dataset, this should not happen")
+ end
+ maxnumlines = ret
  maxnumlines += islast ? 0 : 1
  else
  nentries = length(keys(newg))

src/misc.jl

@@ -142,9 +142,9 @@ function uintofsize(sz)
 end
 
 function to_uint64(bts::Vector{UInt8})
- bts2 = [bts; zeros(UInt8, 8-length(bts))]
+ bts2 = append!(zeros(UInt8, 8-length(bts)), reverse(bts))
  u = zero(UInt64)
- for b in reverse(bts2)
+ for b in bts2
  u = u << 8
  u += b
  end