Multiple optimizations

[cataphract]

• Introduce two types to aid efficient serialization: MapIterableWithSize and NativeStringAddressable. The first is an entry iterable that doesn't need to be iterated twice, as it has a size(). This alleviates the need to implement Map. The second is a CharSequence that also provides a nul-terminated byte buffer with the UTF-8 representation of the string.
• Always allocate the initial object in the beginning of the first segment. On a second WAF run, the initial object is replaced. This is allowed by the WAF (the root element, and only the root element, can be replaced between runs), saves a little bit of space and avoids creating a new ByteBuffer slice() on each run.
• Save memory by not caching PWArgsBuffer objects. To avoid creating many of them, they are reused when possible.
• Reduce the number of native calls to getByteBufferAddress.
• Introduce a memory stats function.

[codecov]

Codecov Report

Patch coverage: 85.93% and project coverage change: +0.89 🎉

Comparison is base (2c2c4dc) 56.16% compared to head (9573836) 57.05%.

Additional details and impacted files

@@ Coverage Diff @@## master #62 +/- ##
============================================
+ Coverage 56.16% 57.05% +0.89% - Complexity 116 127 +11 
============================================
Files 31 31 Lines 2767 2836 +69 Branches 638 651 +13 ============================================
+ Hits 1554 1618 +64 - Misses 762 763 +1 - Partials 451 455 +4 

Flag	Coverage Δ
helper	57.05% <85.93%> (+0.89%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/main/java/io/sqreen/powerwaf/Powerwaf.java	85.24% <ø> (ø)
src/main/c/powerwaf_jni.c	52.33% <68.00%> (+0.21%)	⬆️
.../java/io/sqreen/powerwaf/ByteBufferSerializer.java	83.91% <89.89%> (+0.76%)	⬆️
src/main/java/io/sqreen/powerwaf/Additive.java	75.71% <100.00%> (+3.65%)	⬆️
.../main/java/io/sqreen/powerwaf/PowerwafContext.java	94.54% <100.00%> (+0.10%)	⬆️
src/main/c/debug_helpers.c	60.00% <0.00%> (+2.22%)	⬆️

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