Introduce new more efficient LittleEndianConvert trait

[Robbepop]

This version is more efficient, smaller and more elegant than the old version.
Unfortunately this is a breaking change for implementers of the old LittleEndianConvert trait, however, since wasmi provided implementations for all necessary types this breakage will possibly be alright.

The new trait definition was a result of the work from #287.
Merging this PR will make the common base of both v0 and v1 implementations of wasmi slightly bigger.

Benchmarks

There was no noticeable difference between the two versions, however, I think that's partially due to the fact that our benchmarks do not really test out the memory accesses in depth.

Before

test bench_regex_redux ... bench:   1,807,313 ns/iter (+/- 73,280)
test bench_rev_comp    ... bench:   1,637,458 ns/iter (+/- 60,874)
test bench_tiny_keccak ... bench:   1,294,044 ns/iter (+/- 59,388)
test fac_opt           ... bench:      21,822 ns/iter (+/- 2,109)
test fac_recursive     ... bench:      23,316 ns/iter (+/- 3,726)
test recursive_ok      ... bench:     601,160 ns/iter (+/- 63,188)
test recursive_trap    ... bench:      78,184 ns/iter (+/- 10,151)

After

test bench_regex_redux ... bench:   1,790,866 ns/iter (+/- 168,485)
test bench_rev_comp    ... bench:   1,633,503 ns/iter (+/- 106,277)
test bench_tiny_keccak ... bench:   1,246,634 ns/iter (+/- 49,975)
test fac_opt           ... bench:      22,422 ns/iter (+/- 674)
test fac_recursive     ... bench:      24,096 ns/iter (+/- 1,068)
test recursive_ok      ... bench:     601,510 ns/iter (+/- 26,461)
test recursive_trap    ... bench:      78,344 ns/iter (+/- 7,130)

[codecov-commenter]

Codecov Report

Merging #290 (d3e0958) into master (aea0581) will increase coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
+ Coverage   79.62%   80.00%   +0.38%     
==========================================
  Files          30       30              
  Lines        4756     4672      -84     
==========================================
- Hits         3787     3738      -49     
+ Misses        969      934      -35     

Impacted Files	Coverage Δ
src/memory/mod.rs	76.32% <100.00%> (+0.45%)	⬆️
src/value.rs	68.84% <100.00%> (+2.77%)	⬆️

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