Reuse allocations for br_table translations

[Robbepop]

This causes less strain on the heap allocator for Wasm binaries with many br_table instructions.

[codecov-commenter]

Codecov Report

Merging #440 (0ab2a1a) into master (11b35ae) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
+ Coverage   79.33%   79.38%   +0.04%     
==========================================
  Files          72       72              
  Lines        6198     6198              
==========================================
+ Hits         4917     4920       +3     
+ Misses       1281     1278       -3     

Impacted Files	Coverage Δ
wasmi_v1/src/engine/func_builder/mod.rs	92.44% <100.00%> (+0.36%)	⬆️

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