Add GGUF support

- Made all the mechanics of grammar parsing (GBNFGrammarParser, ParseState) internal. Just call `Grammar.Parse("whatever")`.
 - Added a `GrammarRule` class which validates elements on construction (this allows constructing grammar without parsing GBNF).
   - It should be impossible for a `GrammarRule` to represent an invalid rule.

Based on this version: 6b73ef1201

- Just convert it to a `string`, nice and simple
 - Write the bytes to a `Span<byte>` no allocations
 - Write the chars to a `StringBuilder` potentially no allocations

- Modified `llama_sample_token_mirostat` and `llama_sample_token_mirostat_v2` to take `ref float` instead of as a `float*`. Less pointers is always good.
 - Modified `llama_sample_repetition_penalty` and `llama_sample_frequency_and_presence_penalties` to take pointers instead of arrays. This allows the use non non allocating types (e.g. Span) instead of arrays
 - Modified higher level API to accept `Memory<int>` instead of `int[]`, which can be used to reduce allocations at call sites

- Used those methods to add a `Clone` method to SafeLLamaContextHandle
 - Simplified `LLamaContext` by using the new methods
 - Sealed `LLamaContext` and `LLamaEmbedder`

- Minor changes to cleanup some of the compiler warnings

Type conversion is now done in the property rather than the utils class and uses the System.Convert class to ensure consistency.

- Deleted `NativeInfo` (internal class, not used anywhere)

- Passing encoding in the `IModelParams`, which reduces how often encoding needs to be passed around

- Converted `SafeLLamaContextHandle` to take a `ReadOnlySpan` for Eval, narrower type better represents what's really needed

- Integrated grammar into sampling
 - Added a test for the grammar sampling

Multi Context

* Added a bool to sbyte Utils convertor

As an attempt to avoid using any MarshalAs attribute for .Net Framework support this Utils method will take in a bool value and return a 1 for true or 0 for false sbyte.

* Changed all bool "MarshalAs" types to sbytes

Changed all previous BOOL types with "MarshalAs" attributes to SBYTEs and changed all the setters of them to use the Utils.BoolToSignedByte() convertor method.

* Fixed Utils bool convertor & added sbyte to bool

Improved the Utils bool convertor just casting an sbyte value to get rid of the unneeded sbyte array and added an sbyte to bool convertor to convert back the way to a C# bool assuming any positive value above 0 is a bool and no bools are packed in the single byte integer.

* bool to & from sbyte conversions via properties

All 1byte bools are now handled where they "sit", via public properties which perform the conversions to keep all external data able to communicate as it did before.

- sealed some classes not intended to be extended

The biggest single change is renaming `LLamaModel` to `LLamaContext`

- Added some more comments
 - Modified `llama_tokenize` to not allocate
 - Modified `llama_tokenize_native` to take a pointer instead of an array, allowing use with no allocations
 - Removed GgmlInitParams (not used)

Minor quantizer improvements

- Added disposal check into `SafeLLamaContextHandle`

- Also refactored it to return an `int[]` instead of an `IEnumerable<int>`, solving the "multiple enumeration" problems at the source!

feat: update the llama backends.

It solves the problem with netstandard2 (is it really netstandard2 a thing right now?)
Change context to solve problems.

5f631c2679

Fixed Memory pinning in Sampling API

- Moved repeated code to convert `LLamaTokenDataArray` into a `LLamaTokenDataArrayNative` into a helper method.
   - Modified all call sites to dispose the `MemoryHandle`
 - Saved one copy of the `List<LLamaTokenData>` into a `LLamaTokenData[]` in `LlamaModel`